It's kind of a tradeoff.  The big things that you may value are:

• Multi-threaded rendering; if you have a multi-core processor, your renders can run much faster, as the renderer can make use of up to 4 processor cores... possibly 4 times faster with the same options, but realistically not that much faster because the way Poser 7's multiprocessing is done is a bit stupid.
• Memory management is significantly better; scenes that would regularly run out of memory while rendering in P6 have behaved MUCH better under P7.

On the downside, new in P7:

• Shadows are kind of wonky
• "Smooth shading", phong shading, doesn't seem to work quite right

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