My impressions are exceedingly positive - I picked one up very soon after release, and just spent two weeks in east Africa with it.
I mentioned elsewhere that I have noticed the "chatter" but not so much that I would have commented on it if someone asked me what I thought of the lens. From what I can tell, it's mostly when there is movement - either from the subject or the lens - and my focusing/long lens practices (using AF-ON) don't seem to trigger it much. It's odd when it happens, but fortunately it doesn't seem to happen much on mine.
Additionally, I have found the AF to be very accurate. In all of the shots I took in Africa, I honestly only had a very few that were OOF, and most of those were scenarios like seeing a small bird amidst branches, whipping around and getting a shot off, and then having it fly. Scenarios where you have a very high % of OOF shots with any lens.
I have not experienced any softness that I wouldn't consider normal at infinity - all lenses get softer at infinity, and it's certainly a lot sharper than my 200-400mm at that distance. It's on par, from what I can tell, with my 300mm f/4. The VR takes a tick to kick in, but that's normal with long lenses.
I have a little dust inside the lens from Africa - not much, but it's there. I assumed that was par for the course in the dust of a safari and a lens that extends during zooming.
What is your experience with other long lenses? I only ask because, for instance, if your only other long lens was the 300mm f/4 + TC, getting the hang of when VR is useful or not takes a little bit.
For birding only, barring the need for VR, I agree that the 300mm + TC is probably a good choice. It's half the price, and delivers very good sharpness (though my 300mm + 1.4x is not as sharp as my 80-400mm). It's sure not as flexible, though.