Upgraded my Orange contracted I5801 to Official Froyo via Kies and, after a few initial problems, am happy with overall results.
Pros:
App to SD: the main reason I wanted/needed upgrade, though surprised by how many apps don't want to make the move.
Faster, smoother and more slick. Noticeably so.
Improved keyboard, including a pointer that will do the job of fat fingers when trying to insert a missing letter in a word or select text to be copied.
Settings icons look better! Woohoo! To be fair, there are some welcome tweaks in settings function, too.
Call answering graphics and response better, along with alarm shut-off: no more pressing harder and harder on the stop button first thing in the morning! Or is that just me?!
Battery calibration improved, thought not fixed. Will need longer to assess any battery life change.
New stock task manager makes uninstalling or killing apps easy. No more Adao tyranny! Kidding guys, your stuff still awesome.
Tethering.
And the cons...
No GPS fix.
No Flash. (Try Skyfire as a browser to see Flash vids, which has a conversion function).
For me, there were glitches after upgrade, even after several soft resets. Apps hung up and messaging a bit crumbly. Factory reset seems to have settled everything down, but that's a pain, as ever, especially if Google account password flees your brain for several hours.
Contacts back-up with Orange unsuccessful. Asks for password and user name I can't remember giving. Website that is back-up for the back-up is "down for maintenance". Most details copied to sim, thankfully.
New stock task manager can't quickly close non-third party apps: needs force close. Trying to run Adao's task app alongside stock made for instability. Stock won.
All bloatware still there, plus something called AllShare, which tells me I can beam content from my phone to other stuff in my house. Initial attempts unsuccessful. All help appreciated, but I think It's steering ppl towards buying a particular type of Samsung TV.