I very rarely use it, so can't say i've had problems with it 'if' the note off option is enabled, of course. Without that it is really bad!
Do you prefer input quantise as you're using REAPER for live work, or is it just a workflow that you enjoy?
Personally, like most people i do prefer to quantise after recording a performance but the fact that there are problems with it, and have been so for so long, really need addressing.
Is this only a mac specific issue do you know?