It crashed half a dozen times shortly after launch for me, as it was grinding away, saying it was converting a session from v4 to 5. I repaired permissions and tried again. It launched and finally showed a few thumbnails, then crashed once more. It now crashes upon launch for me. I followed Jack's advice and trashed everything I could find from version 4 before installing. I see that a number of people seem to be faring alright, but it is not working for me on a MacBook Pro running 10.5.8.
Edit: I repaired permissions again and it has stopped crashing on launch, but now it says that there was an error connecting with the activation server. It shows that it is activated, but my activation history has disappeared. I don't know if my earlier problems might have been related to this or not, but I'm thinking that they may be having server problems beyond just the user forums at PO. The initial activation connection seemed to work fine, but that was an hour ago.