Hope this helps. CS5 is a huge upgrade for Mac users, with a 64 bit version of PS. This alone is worth it, if you have the computer horsepower.
I have used every version since PS3. I have already updated to CS5. Only used it for 3 days now, and its a small upgrade for Windows users, so far. In all honesty, the Bridge Output, being able to save templates and apply my logo to each PDF is quite good. The mini Bridge is cute too, but we will see if that helps. The 'build and export cache' now works like a charm, finally. This is great news for giant databases. A really nice feature that now funtions perfectly.
Outside that, ACR may have some processing upgrades, but no new features. I use ACR only for RAW processing and already like it a lot. The ACR noise upgrades are nice for high ISO users, but again, I shoot native ISO and rarely need that upgrade.
In PS, some of the content aware heal-fill upgrades, HDR, are nice for some photographers, although not for my work. Maybe the new refine edges will apply to my work. But that's it - for this photographer.
Although I will always upgrade (and did), I would rate CS5 as an A+ for Mac users and a C for Windows users. This upgrade is all Mac.
I would guess Adobe might agree too.