I have my iPhone 3GS on the latest firmware iOS 5.1.1, & I can safely tell u that the performance is very good. Notifications works very well, seldom lack in the interface & safari loads very quickly. Overall, the experience is a very decent one. However, I've heard some 3GS owners complaining about everything being very leggy & such.
From my point of view, I strongly encourage u to update to iOS 5. Many new features that enhances the user experience!
it should be fine, mine works fine at 5.0, i expect 6.0 would be similar, too bad the phone still sells for $375 unlocked from Apple, more expensive than Galaxy Nexus And used once can easily have all kinds of cracking problem and battery problem consider how old it is.
My first 3GS that I got a few months after launch was updated to every OS up to 5. It was very sluggish and I wanted to do a clean instal but never got around to it. Well, I broke my phone by dropping it and borrowed an 8Gig 3GS from a buddy who wasn't using his so I did a clean restore/instal on his. Marked improvement was noticed, still had some slow downs but not as often as I did before.