One more set of benchmarks to add. I don't have 5.5, so can't run the desmonema scene. I created a simple quick scene with default B5 sky with 5 large metaballs,  4 of which used transparency.

My config: dual pentium4 3.00GHz, Windows XP Pro, Bryce running standalone

B5:

Normal anti-aliasing, fast preview mode 2:13

Normal anti-aliasing, regular mode 4:02

B6:

Normal anti-aliasing, fast preview mode 1:26

Normal anti-aliasing, regular mode 2:16

That works out to 35% increase for fast preview, 56% increase for regular. CPU was at 48% for B5 & 99% for B6. Also noticed that file size decreased - B5 was 135kb, B6 was 74kb. I'm not particularly interested in using Bryce for creating realistic scenes, so I'm not sure that HDRI will have much value for me, but for the rendering speed improvements alone, I can't really think of a better value for $6 (and I also pick up the vastly improved preset library management - which I believe was introduced in 5.5?)... 

"Every child is an artist. The problem is how to remain an artist once we grow up." - Picasso