I have been using Poser 5 in Mac OSX (10.2.8) since P5 came out for the Mac. I have been very happy with it and had very few crashs all though part of the reason may be the fact that I have 1.5Gb of RAM. 1) P5 is more stable the P4 in the classic mode. 2) P5 has far more features then P4 3) P5 is slower then P4 4) P5 is a good excuse to get a G5(Come on tax refund) If you do get P5, max out the RAM in your iMac. P5 loves RAM! The service release 4 is for Windows only at this time.