A few weeks ago I purchased the Tamron 70-200 f/2.8 lens to replace my trusty Minolta 70-210 f/4 (beercan) lens. The old f/4 was reliable but I found it was just not very good in high contrast situations wide open (blooming occurs). I've had my eye on the Tamron ever since it was announced back in February or so. Overall, the lens doesn't really disappoint.
On a Sony body, the lens' autofocus is driven by the in-body motor. On the a700, the motor is very fast and torquey, and the lens responds in kind. It does take a long time to rack back and forth from infinity to minimum distance (a focus limiter would have been very useful) but overall actually acquiring focus and tracking seems to be overall very good. For a Canon or Nikon user this is going to be a very tough decision to make, because while Canon and Nikon's f/2.8 options (with and without IS) cost quite a bit more, they do have rock solid ultrasonic motors. There is also no IS, which for a Sony or Pentax user is not a problem as they have in-body IS. There is also no full-time manual override, instead you pull the focus ring towards you to enable manual focus. It works well enough.
The actual optical results are very hard to argue with. I have a few shots in the queue from last week where I used it to watch some ground action at BDL. I was out at BOS today watching actual moving stuff and it worked very well. I've used it indoors for concert photography (along with strobes) and I've noticed that at 200mm f/2.8 at close distances the depth of field is so narrow that you really need to stop down to f/3.5 or f/4 to get someone's face in focus for instance. The lens is still sharp at f/2.8, though, and when the subject is far enough away (e.g. in your standard portraiture) the plane of focus issues are less of a problem. The lens is very sharp, has low aberrations (both distortion and vignetting on APS), has good background blur... overall an excellent performer.
It feels pretty tough even though parts of the lens barrel are made out of plastic. The tripod collar is easy to remove and feels pretty substantial. The lens hood is large and deep, though I wish it had a sliding window to adjust a polarizer. The lens is built to a price, and for a $699 entry price it had to cut a few corners (namely one section of the outer barrel made of plastic). It does come with a nice padded soft case.
The lens is a solid option for a 70-200 f/2.8 and if you don't feel like spending big bucks on OEM glass. For Canikon users it's a much harder decision to make as the in-lens motor is not as good as in-body options on the screw driven versions of the lens. I have not tried a teleconverter with it, but I'm sure a quality 1.4x will work well with it as the lens has sharpness to spare. Though I will mostly be using this lens for non-aviation purposes (where something lighter and more practical than the Bigma fits), it'll perform well if you're close enough that 300mm isn't necessary.
[Edited 2009-01-17 14:32:08]