Althought I own Poser 7 since it came out (x-mas present from my husband g), I´m still using Poser6. In the moment P6 is running without any problems and he has nearly the features as Poser7. I always render with Firefly, and from my point of view, I like it more that I can set my own preferences (texture size and so on). With Poser7 I can´t as so many things are just "ready" for the user. 
Poser 5 I only used for a very short time. I wasn´t happy with it at all becourse I wasn´t abel to change textures (manual) and so on at all. Using Poser6 I could vote between the Poser5 screen or one which is familar with Poser 4th. Since I can change textures without Inj-files again.