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. Have you seen games running on 3GS vs. 3G?
Not to mention I believe the 3GS is going to blossom a bit more with future software updates and apps that take advantage of the new hardware.