I just upgraded to the 64GB 4S a couple of days ago. Really pleased with it
It's so much faster than my old 3G. The camera is very good for a phone camera and the audio quality has also improved - it's hard to tell the difference between that and CD's now, even through a pair a studio reference cans. I've not had the chance to try out Siri yet but will have a good play around with it over the weekend. It's definitely a worthwhile upgrade IMHO.
If anyone is interested in a 16GB 3G I have one for sale. Pretty much mint condition - it's been in a case all of it's life. Yours for £90. PM me if you're interested and I can send you some pics.