I use Tasks to track both development features and bugs. The heirarchical organization makes sense to my mind and I like being able to see the tree of an entire release (features and bugs). I've used Tasks Pro for this with several small development groups and the net result was much more voluntary usage of the system than I saw with the bug tracking systems at several larger companies I was at. I think the 'thousands of drop downs and too small text fields' approach you see in many bug tracking systems makes them so slow and painful to use that many developers feel like they are a waste of time.