The biggest complaint I’ve heard about SP6 is that “Save to Excel 2007” doesn’t work. I believe the problem only occurs when upgrading from SP5; a fresh SP6 install doesn’t have the problem.
I went from FP3.1 to FP5.4. My original plan was to go to FP5.3 (IIRC), but there were several unresolved bugs in the calculation engine. FP5.4 resolved most, but there were still some “changes” that required modifications to reports. SAP has a pretty decent wiki regarding the known calculation engine changes. There were several changes in the “Where()” logic and with merged dimensions, so that is likely the reason why you’re seeing different results.
For what it’s worth, we discovered several new bugs in FP5.4 that are resolved in FP5.6. I’m not sure how that lines up with the SP6 track.
As far as knowing what’s different, the best source is the Fixed Issues list that’s updated with every release. I maintain a post here to keep track of the release notes, and I’ll update it again after the next release.
Joe
joepeters 
(BOB member since 2002-08-29)