Well, only since you asked....

I have Poser Pro, don't have Poser 8, but I have a girlfriend who I talked into buying it a month or so ago. (and I feel really bad about that now...)

So having had a few chances now to check out Poser 8, IMHO, the only thing I see better about it over P-Pro is better optimization of the way the CPU is used during rendering. All cores at all times.. although IIRC, shadow maps are only calculated on one core.

Aside from that...
The new library is problematic, to put it politely, even with SR1. It's especially a PITA if the machine you want to install it on isn't connected to teh interwebs, so you can easily and quickly download the latest Flash plugin for Internet Explorer. There's a lot more I could say about it, but I'm not going to. Suffice it to say I don't like it and I think it was wholly unnecessary. If you're used to the traditional Poser runtime scheme, you won't miss not having the new and... err... improved Poser 8 Runtime Debloatifier.

Also, there's a definite performance hit on the OpenGL response in the viewports. And before anyone suggests use a better video card and update drivers, I built that PC for her and it has a GTX 260 in it, and yes, the drivers are fully updated. Compared to Poser Pro, the OpenGL definitely seems slower and less responsive. On large scenes, that is. The OpenGL seems to bog down more, the more complex a scene gets. Yes, that's normal, but it seems Poser Pro overall is able to handle more stuff in a scene and keep it moving along.
Of course it's also worth mentioning that Poser Pro has 64 bit rendering, which Poser 8 does not.

Now, Poser 8 has IDL - a form of Final Gather GI. Which would be good if they had optimized it better and implemented more controls over the settings, but unless you want to spend hours to get minimal quality GI and even more hours to get slightly better quality GI... well, you know. ;-)

It does have better light falloff, now that you can use inverse square on the lights. That's a major improvement which was a long time coming, so kudos to them for doing that.

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I'm assuming that Poser Pro 2010 will have all the features that Poser 8 has, plus some, so I'm waiting to see what that's all about. And so if you're considering either Poser Pro or Poser 8, personally I'd recommend just waiting to see what Poser Pro 2010 has to offer. I think it's supposed to be out either late this year or early next year.
Then again, the current Poser Pro is a really nice program, IMO, the best Poser yet.
Hopefully they'll do something about the library in Poser Pro 2010. A "pro" app, after all, doesn't really need a built-in runtime debloatifier, being that "pros" probably already know how to manage their runtimes and don't need the extra...err... "help"...
And hopefully the GI in Poser Pro 2010 will have a handful of new settings in it to be able to optimize GI better for quicker renders.