I've been using Timelog for over two years and I can't recommend it highly enough.
Prior to using the software, I reviewed many time tracking software such as Timecache, iBiz, Studiometry, Officetime, Timebox, etc. Even tried a couple of web-based time tracking services.

The thing that stands it apart from the rest is a combination of a simple interface with a rich feature set. The 'iTune' like interface makes it really easy to drill down to find the set of information we need. We find the ability to enter time via iCal really useful. Simple and user friendly to enter and the data can be shared amongst the six users in our architectural practice. All of us have real time data of time spent on a project / phase.

On top of all this, the support/response I have received from the developer has been exceptional. Do yourself a favor and try this software.