Poser 6 and later really come into their own when you have a dual monitor setup. I run Poser 6, 7 and Poser Pro on a dual monitor machine, one monitor dedicated to the document window, one monitor dedicated to the library, editor windows and palettes.
Raytraced lights now can have soft shadows, a major omission in Poser 5
Poser 6 and later have "official" support for MAT poses - they're called Material Collections, and you can create those within Poser itself.

Don't buy Poser 6 for the new figures - James (human male) is pretty OK, but Jessi (human female) has some major flaws - forearms far too long, upper arms far too short, ridiculously large hands and feet. Poser 5 Judy may not be a beauty queen, but at least she's got some realistic proportions.
The basic environment figures and props are pretty good (after all, it's RDNA MicroCosm), but they come with very few morph and material options. If you want some real use out of the environment stuff, you're better off buying the complete thing at RDNA - oodles of morphs, plus an immense variety of textures and materials.

Python support within P6 is also improved. You can find quite a lot of P6 or higher Python scripts here in freestuff. Some of them can do really complicated stuff (shameless plug, but I am still quite proud of my SpawnCharacter Python script. A script that fully automates INJ/REM pose file generation from a custom morphed character).

$30 is a bargain.

The pen is mightier than the sword. But if you literally want to have some impact, use a typewriter

My gallery   My freestuff