I paid 499.99 and sold my 3G for 350.00. I think it is worth it, this phone is more more enjoyable to use now. It's much faster which quite frankly is reason number one for me, I mean it is just A LOT faster, in everything.
I used to get annoyed with my 3G, as I am VERY impatient, the 3G just lagged in things (sms, browsing, opening menus at times) and I would find myself just.....annoyed.
The speed combined with the new camera functionality and compass(better GPS) make it worth it for me. I mean have you seem games running on 3GS vs. 3G?