The 5870 is a great card, no question about that. It's the fastest single GPU solution, it trades blows with top dual GPU cards while maintaining a decent profile in terms of noise and power consumption. It improves on several features, and it brings DX11 to the table. It also carries a decently modest price tag. So, is it good? Yes. Is it spectacular? IMO not quite.
Am I going to be an early adopter? No. Why not? Several reasons:
1. I'm going to wait a bit and see whether drivers are mature and stable enough.
2. 5870 will come in more exciting versions, I'm sure - factory OCed, with custom cooling, 2 GB RAM, etc.
3. If the GT300 is released soon and outclasses the 5870 (which I consider possible, despite all that rumormongering), it would be quite silly to buy a card now and another one in e.g. 2 months.
4. There are very few games that I can't "max out" (1920*1200, at least 4*AA if supported) on my current card, the GTX 280... and games which need an fps boost actually need these fps in spades so an upgrade has to be really huge, and the 5870 doesn't seem to provide that.
A fantastic card for people still riding 8800 GTXs and whatnot. Also, an understandable upgrade for performance freaks sporting SLIs and 30" displays. For a demanding gamer with a 1920*1200 monitor and an already strong card like the 280 who is more concerned about real world performance than pretty benchmark numbers, not necessarily a revelation.
However, If I had to buy a card now, it would be the 5870, no doubt about it.
BTW, I'm not a fanboy of any chip maker. Whoever builds the card that constitutes the best package of performance, image quality and ergonomics (which for me includes stability, noise profile, thermal characteristics), gets my money.