there seem to be 2 differences between 8.1 and 8.5, genesis compatibility and soft body physics.

• if you have any version of Poser with dynamic cloth then the soft body physics probably isn't worth it because of the time it takes 8.5 to do the calculations and the superior finetuning that Poser can do.

• if you are satisfied with gen4 figures then genesis usage isn't terribly important.

having said that, I am very happy I upgraded to 8.5 even though I continue to use gen4 figures more often than not.  perhaps only because reading complaints set my expectations very low, I have been extremely surprised with how well genesis and genesis 2 work in carrara, in a good way.  It is very easy to use genesis and genesis 2 to make a morphed person or creature in carrara and use content that I already own with the new shape.  i have even been able to use posers wardrobe wizard to adapt poser 6 and 7 clothes for genesis 2 characters.  

• so, I fully recognize the probs people continue to have with genesis in carrara, and I use poser for draping not 8.5, but for me buying the 8.5 upgrade has turned out to be a very satisfying decision.