Ok, I started having weird issues with 5.0 on my 3GS...
- Taking a Screenshot would show Volume for some reason.
- Images shown as thumbnails in the gallery weren't matching up to the correct image (a thumbnail of a picture of a tree would actually be a picture of something else when you tapped it).
- Performance started to really lag with Safari, lots of delays opening programs, etc. basically, out of RAM all the time.
- Mail kept downloading 50 emails over and over and wouldn't display anything.
- Mail kept making iCloud my default account (even though Gmail was made my default).
Basically, just normal "it's beta, not optimized, and generally doesn't work right" stuff. Plus, I was missing all my fun jailbreak apps.
I wiped it and went back to iOS 4.1. Despite what Apple says, you can still go back to an older iOS after installing 5.0.
On a clean boot, free RAM on the 3GS w/ 4.1 hovered around 150 MB (up to 170 MB when you free memory with SBSettings).
On a clean boot, free RAM on the 3GS w/ 4.3.3 hovered around 90 MB.
I can only assume 5.0 had even less RAM available. Besides the new Notifications/Widgets thing, other stuff is designed for devices with more memory. Mail will cache 50-1,000 items (4.x would cache 25-200), the Newsstand widget is _always_ loaded (I didn't see how to turn it off).
I'm looking forward to the next beta. There is still a chance that 5.0 will run as well as 4.3.3. (I don't expect 4.1 or 4.2.1 performance, though.)