I have tried almost every single GTD soft out there. From macs things, through action complete, thinking rock, gtk gtd and many more. This one provides tbe best balance between usability, features and development. And of course price - you can not beat FREE. Dev team is extremely patient and they are very determinated to make this software the best.
[Edited by lebaux, November 03]
My previous excitement is over. App is now paid (but 20 bucks/year is a fair price!) if you want full sync options. Dev team is not responding very well in forums for requests - like the topic about export. After two years they have just closed it http://support.doit.im/entries/308767-export-or-print-view. Twitter is the same, they are responding but only PR and marketing bullcrap. Blog is filled with semi-useless tutorials, no info about the development of anything. I feel that they are group of stubborn chinese guys who are just doing it their way no matter what. Don't get me wrong, they made a great GTD/TODO app, but I am not so happy with it as before, because they do not hold onto promisses. This is however just my opinion. My recommendation here would be DGT GTD http://www.dgtale.ch/index.php?option=com_content&view=article&id=52&Itemid=61.