Tried so many apps, never really got settled, I think this one will be the one for good. I'm loving it so far, it's clean, easy to use, and yet very powerful. The client handling aspect is just too epic.
Easy to use and featured service. To be completely satisfied I recommend everyone to use it with TMetric time tracker. TMetric has a convenient integration with ActiveCollab (works just in browser as an extension), so you won't have problems with customization, just start track time inside ActiveCollab.
I've been using ActiveCollab for quite some time now, and it really helps me organize my work and projects. I love the fact that I don't need almost any other tool to run the daily stuff, since everything can be done through AC, from the beginning to the end of the project.