P8 works great, I haven't touched PoserPro since I installed P8.
PoserPro renders larger scenes, but P8 comes a long way in that direction too - much larger as Poser 6.
Indirect lighting makes it much easier to set up lighting a scene - especially with outdoor scenery. Often 1 or 2 lights are enough to get great results. PoserPro does not have that yet.
The new UI is a matter of taste. For me the library is working much faster as the old one since you can directly access the folders and items you want without going up and down through the runtime hierarchy, The improvements in SR1 makes the icons larger and easier to use. New improvements are promised for later releases.

For me - and many others - Poser 8 is stable and and is a great improvement over Poser 7 and - depending on your needs - over Poser Pro.