Well, P5 is very buggy, it never worked on my computer. I have P4, ProPak, P6 and P7, but time after time when doing my artwork and creating content, I do it in ProPak.  I love it, what can I say?  I'll use P6 if I'm going to use shaders or create P6 above content (but will always include ProPak stuff too) etc....but I use them few and far between. 

I'm a Brycer from way back and honestly, if I'm going to have to wait hours for a render, I'll fire Bryce up and render in there :)  What can I say?  What I do find interesting is the morph tool in P7, but haven't had too much time to learn it. P7 is still way buggy too,  I like all versions but ProPak is my definate favorite....
I think it just depends on what you want from the software.