Although there may be tuning issues making it worse helicopters of any type have a problem descending quickly unless there is sufficient forward motion to ensure the blades aren't descending into their own prop wash. If you descend too fast without enough forward motion the copter enters "ring...
Fix has been on the way for quite some time. However, the wii-esc firmware solves that problem and really works well. Available here: http://code.google.com/p/wii-esc/wiki/About?tm=6 . Doesn't have all the bells and whistles of Simonk but what more do you really need other than it works well?
You said you wanted something you can tinker with and improve upon. Definitely can't with DJI. OpenPilot CopterControl is very feature limited. It's a sport copter controller - not an autopilot. That leaves MultiWii and Ardupilot (there are others but more obscure). Both have very active dev...