Please keep in mind I only picked my body up on Friday afternoon and haven't had a lot of time to play with it yet. But these are my initial thoughts...
*How's the AF in very low light? - The centre AF point is great (comparable to a 1D3), the others are pretty ordinary. I gave up on the outside AF points in low-light, but there's nothing to fault about the centre point. Of course the "cropability" of a 21mpix image means that you can shoot away with the centre point and then compose your shots in post using the crop tool and still end up with a 10+mpix file, not that you'd want to do this with every shot you take.
*Is the high ISO performance what you expected? - Yes, I think maybe?!? I'm still a bit undecided as I've seen conflicting results, but I can definately say it isn't worse than the 1D3. I did some 6400iso test shots comparing 5D2 to 1D3 with identical settings and all NR switched off and there wasn't much difference, but I have a few real-world 6400iso shots from the 5D2 that are much better than any 6400iso shots I've ever got from a 1D3 (better than I expect at 3200iso). I think there's more to this story than first meets the eye, and some of my testing was in unusually high temperatures (38degC) which affects noise levels and may have skewed the results.
*How's the metering? - Great. Really great. In low-light I think it's considerably better than a 1D3. In bright light I think it tends to over-expose just a touch for my taste. This is personal preference however, and I haven't shot in a wide variety of conditions so it's really a bit early for me to tell.
*If you owned another EOS, what was it and how does the 5D2 compare? - Compared to my 30D, there is no comparison. The 5D2 simply destroys the 30D. Compared to my 1D3 it's a tough call. I think the files the 5D2 produces are much better than the 1D3, but the 1D3 is a nicer beast to use. Obviously each body has features that the other doesn't (21mpix + video VS 10fps and 19 great AF points). I'd say they are equal, but not the same. Of course the perfect combo is to have both a 1D3 and 5D2 - you can have the best of both worlds while also having a backup body.
*If you paid full price plus tax, after shooting with it, do you kinda wish you'd have waited until you could get it a little cheaper and tax free? - I was lucky enough not to pay full price (I paid AU$3579 = US$2390, while RRP here in Aus is AU$4200 = US$2800), but I think it is worth every penny and then some.
*Is this perhaps the best DSLR you've shot, by this I mean, everything is bang-on period, no regrets? - I love it, and certainly have no regrets. I love the video quality and this is something I've been waiting desperately to play with, but my only disappointment with the camera is the lack of manual controls for video. I really REALLY hope there is a firmware update that gives the user aperture/iso/shutter speed controls when shooting video.
* If you owned a 1Dsmk2 or 5D, does this new 5D2 smoke your old bodies as far as IQ? - The IQ certainly beats the 1D3 ("smokes" might be a bit too strong), but there will be times when the 1D3 will get the shot that the 5D2 can't. As I said, I think the 1D3 and 5D2 are about as good as each other; just designed for different purposes.