Poser 4 and Pro Pack run fine in OS 10. I've switched to Poser 5 and rarely run Poser 4 in classic mode anymore. There are some Python scripts that will only run in Poser 4 so I keep it on my Mac just in case (a tip of the hat to Ockham by the way for all the brilliant Python scripts he makes available). Poser 5 sometimes runs dog slow, even on my dual-processor 2 gigahertz G5 with 1.5 gig of RAM. I don't even bother with the Hair Room as it's painful how slow it can be to do some simple tasks. I keep waiting for Curiouslabs to release an overall speed fix for Macintosh(and hopefully make the Face Room less clumsy at the same time). But in general there's so many cool features with Poser 5, like reflections, volumetric lighting, being able to use movies as textures, etc.