i've installed ios5 on my 3gs and it's responsiveness (sp?) has gone back to the way it was before. On 4.3.5 the phone was sluggish and unresponsive however it's back to the way it was when I bought it.
I like the access of the camera from the lock screen. Double tap the home button to bring up the Camera icon and also you can the the Up volume button to take pictures.
I like the find my phone feature. This was not freely available to 3GS users until now.
imessages might be useful for MMS messages and sharing of photos. From what I can gather alot of the mobile phone companies are now charging for MMS messages and not taking it out of the monthly allocation of text messages that is part of a contract.
I'm underwhelmed by the other features thus far. Notifications will probably be useful over time. Not that fussed about icloud as I don't think 5Gb is large
ios5 works well so far on my 3gs. Just took ages to update.