Yes, I'm happy. I kept the iPhone 3Gs and skipped the 4, so this is a pretty drastic upgrade for me. The speed is much better, both in terms of processing and 3G performance. The screen is nice, but seems a bit more difficult to read outdoors than the 3Gs.
The camera is a huge improvement over the 3Gs, both for stills and HD video recording. I bumped up to 64GB on this one, so I'll have room for HD video.
Siri is mostly for fun right now, but I'm slowly learning how to integrate it into my workflow. I played with dictation to reply to messages & Facebook. It does very well learning words, though I need to know if I can get it to include punctuation.