I have an 8 GB first gen iPhone that I got in December, the day my Sprint contract expired. I picked up a 16 GB black iPhone 3G yesterday, but after only a day, I'm starting to think about taking it back and going back to my first gen iPhone.
Honestly, I think the only thing I'll really miss is the extra 8 GB. I live in San Francisco, but the 3G speeds haven't been phenomenal, and the pinpointed GPS is not really any more helpful than the faux-GPS of the original. On top of it all, I find that I miss the design and heft of my original.
I only hope it's simple to return the 3G and go back to my original...I realize I'll have to pay the 10% fee, but that's only $30.