Kimai tries to be both straightforward and complex - something quite rarely achieved in programs. One has to take some time to trial-and-error (documentation is rather concise) to figure out how some elements of the interface works, but than it is seems to be just great! Unfortunately, the mobile interface has to be installed separately, although mobile devices are quite wide-spreed today (and will more).

In nutshell: nice, one-task, useful software.