OSOS10  I finally upgraded from v5.2 to v6.0.3. Indeed the new version is a major improvement. Particularly in the user interface, a vastly improved input/output formats, graphics and more (this is important when accounting for the fact that the set of improvements during the shift from v4.X to 5.X was essentially an empty set). Thus I find Mathematica 6.0.3 working smoothly. The only drawback is that under OS10.5.5, whenever Mathematica is running, my MacBook doesn't sleep and hence my battery is being drained. I hope that this will be fixed soon (before I have to purchase a new one).  
(Version 6.0.3)

PANTHERA  Too bad "leech" is already in use as an application name because with regard to upgrade fees, it would be an apt moniker for this app.  
(Version 6.0.3)

BLULOO  Nice program but why do they continually charge upgrade fees for point updates/bug fixes?  
(Version 6.0.3)

DGEO  It must be stated that Wolfram has discontinued 64bit PPC support for Mathematica 6, it just runs in 32bit now. Mac PPC G5 is the only 64bit platform that Mathematica runs in a 32bit constrained mode. If you really need 64bit and have a PowerMac you should stay with the previous version 5 ... or look elsewhere.  
(Version 6.0)