In my opinion Poser 12 is is a Work In Progress at the moment. It apperas to be pretty stable but lots of things don't work quite right at the moment. In general all Python scripts require an update and lots of materials need some reworking. Expect a lot of updates (of both Poser and a lot of content) in the next couple of weeks and months. Jumping on the bandwagon now means you can enjoy the fun of working through the problems and maybe have some influence on the solutions.

If you just want to create art (or other work) pay the money to get the upgrade (relatively) cheap but wait until early next year to install it. By then most issues should be fixed and using it will be less frustrating.

It's looking promising but too many issues at the moment. Fun to join the hunt for the issues but not really useable to do real work.

If you're new to Poser and can't afford to buy Poser 11 and the upgrade you could buy Poser 12 and start learning with that (it does work well enough for that purpose). Otherwise stick with Poser 11 and upgrade later.

Faster rendering and modern render nodes will always be better in the end. The new Python is a requirement for the future but a bitter pill to swallow at this moment. It is a big step but that happens after standing still for 5 years.