There will always be bugs in the first release of any upgrade of any program. With that in mind I always install an upgrade next to the old version. I always keep working in the old version and  I try out some new things in the new one to see what's worth. If you make production work for a living, stay with the old version untill there's a stable service release.
Last year I spent €2500,- on an upgrade for Autocad (2006LT => 2007 full), but I'm still working with the old version, not because the new version is unstable but I haven't got the time to convert all my custom menu's and libraries.
There are some reason why I buy a new upgrade for poser now and don't wait untill it's stable:
First: I'm curious, which is for me the most important reason; second: there is some profit in buying before it's released, like extra content;  third: service releases are always free, fourth: I' will always be  (at least trying to be) enthousiastic about the new features so the makers will know they've done a good job and keep them going, and fifth: I will always complain about the new features so the makers will know that they still have to improve them and keep them going. (I think it's called feedback.)

I'm convinced that if everyone waited to buy untill  the first service release came out, there wouldn't be a service release at all.
Perhaps E-frontier should change their vision about the beta-proces and let more people get involved with, instead being so mysterious about their new release (wow, we've got seven secret reasons for you to buy Poser7, and we will reveal them one by one)  Perhaps they're afraid that someone runs away with their secrets but I haven't seen any fresh and amazing ideas, that other companies could use:
    -multi-threading rendering; well, it's about time they include that.
    -multiple undo; another overdue item
    -talkdesigner; I've seen something similar before, but I don't remember where.
    -layered animation; also nothing new.
    -morphing tools; ditto
etc.
Don't understand me wrong; all these add-ons make it a better product, but they're not really  surprising.

I love the way Rhino3D (McNeel-associates) involves its users in the beta-proces, the beta-version is free for every registered user and there's a newsgroup-forum to report bugs, wishes, results , questions with a load of experienced and enthousiast users and developers who can reply. Every beta-version expires after about 2 months, but there's a new release very two weeks. I know I won't be surprised when the final version will be released (perhaps the price will), but I was surprised several times during the development proces and that makes it fun.

-How can you improve things when you don't make mistakes?