I got rid of my 3GS in August after listening to the hype surrounding the Galaxy S2, it was the same price as an iPhone 4 but a more modern device. I am very disappointed with it.
First thing the hit me was the bright screen, but then if I turned screen brightness up full on my old 3GS it was the same difference, I really didn't see what all the fuss was regarding the screen.
Next thing I noticed was just how weak the WiFi was, I literally had to be standing beside the router to get a full signal and a web search revealed loads of complaints about this.
Then we went onto the camera 8MP WOW, but it reality it was OK, I have taken photos on par with my 3GS, it also has 1080p HD recording which will chew your memory at a disastrous rate, 10 secs = 100mb
I'm a music fan and iTunes love it or hate it was a fantastic system, getting my iTune playlists onto the phone was a disaster but got it on with iSyncr only to discover the SGS2 doesn't read AAC files, well it will read them but all meta tags are unknown. There are work arounds that half solve the problem but in the end I had to convert all my music to MP3, then comes listening to music, disaster, at full volume its about half that of an iPhone meaning you can clearly hear ambient noise, I tried various work arounds non of which really worked.
Then onto battery life, My 3GS could easily last a day and a half, 2 days on Airplane mode. This has to be put on Airplane mode just to get through the day.
A month after getting it I said enough was enough and went to sell it, While both the SGS2 and iPhone 4 are the same price retail for resale the SGS2 = same price as a new 3GS or a poor condition iPhone 4, I put it on Gumtree not 1 phone call, Put it on eBay my 3 week old 32GB SGS2 went for the same price as a new 32GB iPhone 3GS or an old used scratched iPhone 4 and in the end I had a none paying bidder.
So it looks like I am totally stuck with this phone, I am hoping maybe the price of the iPhone 4 drops after the 5 is released but in my opinion the SGS2 is vastly over rated.