This months OPM has a demo of this game and it looks like it's going to be real solid. I played it quite a bit last night and it definitely looks like what you might expect from Rainbow as far as the real sequel to ATV2. ATV3 sure wasn't it, like you said.
From what I saw in the demo, it looks like it's ATV2 with more variety, which is cool. The graphics look about the same, maybe a little better. The handling is just as good as it was and they didn't seem to go nuts with the speed like ATV3 did.
After playing the demo, I'd say this is definitely a must buy, for me anyway.