Hello.
First of all, let me say that I've been using ATK for only few days now, while my co-workers have been using it for almost 6 months.
ATK Framework is really, excellent framework - but with quite special purpose.
If your looking for framework to help you build Administration panel - ATK will save you loads of time. You only have to write few lines of code to build complete and really good administration panel.
The only thing I didn't like about this framework is that you can't really control everything (things going under the hood) and the fact that it doesn't have support for UNIX TIMESTAMPS (I've tried to generate date field but it didn't want to accept time stamp so I had to change some things, add a class).
ATK community is really great, and with my co-worker helping me - I've learned lots of things about this framework in only 3 hours.
HOWEVER if you're looking for universal framework to code your whole site - you might want to avoid this one.
Personally, I'm going to go (really soon) for ATK+Zend Framework combo - ATK for backend and Zend for frontend.