Since SR2 Poser 6 has been rock solid for me, and with SR3 I haven't noticed any difference.
(I'm not even sure what SR3 was supposed to do, as I haven't noticed any difference.)

I agree with Blackhearted that it would be nice if the joints would bend more realistically.
Even poses that I can do with my old and stiff body look a mess in Poser, especially
sitting poses, with the hips and thigh getting all scrunched together.  Elbows never
look quite right either when you bend the arm too much.

I do like what they did with the cloth room.  It is far more faster and stable.
With some clothing items the animation progresses in almost real time.