I know no other bookmark service with Xmark’s cross-browser-sync abilities
Import and export features are good to change to FireFox Sync or Google Sync, but if you use more than one Browser and a mobile device, Xmarks was the only service, which was able to keep all the bookmarks on different browsers and on mobile devices really synced.

has frequent error and cause data loss

Was a very happy XMarks user pre the LastPass takeover and was one of the users that indicated I would pay an annual fee for the service. After the LastPass takeover I paid the fee and on changing my phone could not get it to work on my new blackberry bold, since then i haven't been able to get it to work on my blackberry torch and now I can't get it to sync between home computer and work computer. I have to say the support efforts have been fantastic but at the end of the day they have not been able to rectify the problems. Very dissapointed and currently have deleted it from all my platforms and devicies and looking for an equivalent product.

Xmarks Finds a New Home in LastPass: Browser add-ons remain free, now 12$/year for the iPhone & Android app
On 2010-12-02 Xmarks has been acquired by LastPass, makers of a leading cross-platform password manager. It’s a great opportunity that ensures the survival of Xmarks as the same service that we know & love. LastPass will continue to build upon the service in the coming months. Lastpass is also committed to keeping Xmarks free while implementing a viable long-term plan. Xmarks is transitioning to a “freemium” business model. The browser add-on and the vast majority of what users have enjoyed remains free. Users can then opt to purchase Xmarks Premium for $12 per year, which includes the mobile phones apps for Android and iPhone and priority support, but unfortunately until now not for Opera Mobile and Explorer Mobile for Windows Mobile, Windows Phone 7 or Blackberry . The Xmarks and LastPass Premium offerings are also available bundled together at a reduced subscription rate of $20 per year.

Now we can hope for a soon development of larger mobile devices support, e.g. for Opera Mobile and Explorer Mobile for Windows Mobile, Windows Phone 7 and Blackberry. Until now on smartphones with WM, WP7 or BB you must use http://my.xmarks.com/mobile/ for mobile access.

(1) http://lansingrocks.com/2010/12/xmarks-finds-a-new-home-in-lastpass/
(2) http://blog.xmarks.com/

Xmarks will continue on with no service interruption: “Xmarks Premium” with advanced features
The Xmarks pledge page, where users could commit to paying $10 – $20 annually for Xmarks, was closed with thirty thousand pledges instead of 100000 pledges until Oct.15th, 2010.
Although the intended target was missed, Xmarks now has multiple offers from companies ready and willing to take over the service and keep making browser sync better and better!
This is not a signed, sealed done deal yet. But with multiple offers on the table Xmarks is pretty confident that Xmarks will continue on with no service interruption. They don’t know for certain how “Xmarks Premium” will play out, but early indications are, that some basic Xmarks features will remain free with more advanced features reserved for Premium users. I would be pleased much about such a good solution for this competitionless cross-browser-sync-service for bookmarks.
(1) Details about the plans for XMarks see http://blog.xmarks.com/

wow, they wanted between one and two million dollars to keep the service running...

"I will commit to $10 - $20 per year for Xmarks Sync but only if 100,000 other people will do the same."

— Xmarks Founders, Mitch, Todd, Dave, Penny, and James

You can vote for a continuation and further development of Xmarks for 7-10$/year on (1)
Good news with hope for a continuation, details read on (3)!
I would pay 10$/year for a cross-browser-sync-ability of a developed XMarks including IE, FF, Chrome, Opera Mobile and Explorer Mobile for Windows Mobile and later Windows Phone 7.
I would pay 7$/year for a cross-browser-sync-ability of a continued XMarks as it is in Sept.2010 including IE, FF, Chrome, without Opera Mobile and without Explorer Mobile for Windows Mobile and Windows Phone 7.

My opinion is, that enough people are ready to pay 7$/year to keep Xmarks alive, because XMarks as it is in Sept.2010 including IE, FF, Chrome, Mac, iPhone and iPad, without Opera Mobile and without Explorer Mobile for Windows Mobile and Windows Phone 7 has worldwide no competitor, which has equal/higher cross-browser-sync-abilities as/than Xmarks.
Xmarks really is unique and actually the best cross-browser-bookmark-sync-service of the world.
If anybody does not believe this, this is indicated by the number of LIKES on (2) in comparison with all other alternatives.
What most people need, is Xmarks as it is now with the hope of a soon development including mobile devices support, e.g. Opera Mobile and Explorer Mobile for Windows Mobile and Windows Phone 7.
(1) http://www.pledgebank.com/XmarksPremium
(2) XMarks
(3) http://blog.xmarks.com/?p=1945

Very good comparison (1) of cross-browser-bookmark-synchronization-services :
It seems, that Xmarks (2) has no real competitor concerning the cross-browser-bookmark-synchronization-abilities.
No other bookmark-sync-service than Xmarks is able to sync Bookmarks (and passwords) across IE, FF, Chrome, Mac OS X10, iPhone and iPad.
Additionally you quickly can share a Xmarks-folder (with subfolders) with your personal internet-bookmarks online as webpage and you can vote and see the average voting of other Xmarks-users for the quality of sites (7).
Further as an alternative to the Xmarks server you can use your own server for your bookmarks, and you also can sync the passwords across all your used browsers and computers, if you want.
Bookmark.com (3) has no Chrome-support and a bad reputation on (4), whereas the reputation of Xmarks is excellent on (5) nearly equal to the excellent reputation of AlternativeTo.net, see (6).
(1) http://en.wikipedia.org/wiki/Comparison_of_browser_synchronizers
(2) XMarks
(3) http://alternativeto.net/software/bookmarkcom/
(4) http://www.mywot.com/en/scorecard/bookmark.com
(5) http://www.mywot.com/en/scorecard/xmarks.com
(6) http://www.mywot.com/en/scorecard/alternativeto.net
(7) http://www.xmarks.com/about/features/smarter_search

I'm in the closed alpha for Bookmark.com at the moment, and it's working great for me as an alternative. No Chrome support, but way better mobile apps. All is not lost for cross-browser sync.

To get around cross-browser-sync most browser and I think all, allows you to import your bookmarks from other browsers this has always been an option for most browsers. For mobile devices I know Firefox uses Firefox Home to sync you bookmarks from Firefox Sync and I think Opera Link has a similar feature IE, Chrome, and IE I believe will all able to cross-sync via mobile devices sooner or later. Look at Windows 7 Mobile, and look at android. I'm not sure but I believe Google already supports cross-sync via mobile devices. If not it will be possible.

Xmarks will be shutting down its free browser synchronization service on Jan 10, 2011
I am very sad to read this message about the end of the most useful online-bookmark-service of the world.
Page (1) contains details on how to transition to recommended alternative services.
For more detail on why Xmarks is closing the doors, please see XMark's blog post (2).
Xmarks’s cross-browser-sync (including mobile devices) is a unique feature, which unfortunally no alternative service can provide:
Firefox Sync (3) does not work with Chrome or IE, Chrome Sync (4) does not work with firefox or IE, and Windows Live Essentials Sync Program is not compatible with FireFox or Chrome.
So we can only hope, that Xmarks finds a way to continue its absolutely unique service without any full alternative, which would cover ALL features of Xmarks.
I personally would believe, that the majority of Xmarks users would be ready to pay similar fees as you pay for iPhone apps.
(1) http://www.xmarks.com/about/shutdown
(2) http://blog.xmarks.com/?p=1886
(3) http://www.mozilla.com/firefox/sync/
(4) http://www.google.com/support/chrome/bin/answer.py?answer=165139
(5) http://explore.live.com/windows-live-devices-and-sync-program-settings-ui

All your bookmarks are backed up automatically every day online
Today I have deleted one bookmark folder by mistake. I was very glad to see, that I was able to restore the complete bookmark folders set from backups, which XMarks makes automatically. I was able to select the date of the backup set. I choosed the backup set from yesterday.
This is one more reason to use Xmarks, and indicates, that it is the best and most secure of all bookmark storing and synchronisation systems.

Xmarks is incredibly convenient. I just wish that it would stop annoying me with extra features like Smarter Search and Site Info on Firefox- I just want it to sync my bookmarks! I have to turn off all the extras every time I install Firefox on a computer (it happens more than you'd expect).

XMarks is the best application to store and synchronize bookmarks on multiple computers very easy. Since 12 months I cannot imagine to use internet without Xmarks and ScrapBook ( http://alternativeto.net/desktop/scrapbook/ ). XMarks manages only links, Scrapbook stores complete sites (with comments, if You want). I combine ScrapBook with DropBox, so I have access to the stored internet-sites on serveral computers. Xmarks is also available for the windows explorer, but for this browser only for private use. XMarks has also a web-interface, which allows to publish and share a choosable link-directory with other people.