Again I know this thread is a little old....sorry, but I did want to add my $.02 worth.
While I haven't used this lens specifically, I'm a pretty big fan of Tamron lenses in general. I've been using the Tamron 70-300mm LD for almost a year now and the sharpness this lens delivers is hard to beat at several times the price!
With that said, because you have mentioned sports specifically, there are a couple of drawbacks to Tamron lenses. As was already mentioned, they do tend to be "slow" lenses. If your shooting in bright day light this isn't an issue, but if you're trying to shoot under night time stadium lighting, this can be a problem. Secondly, most Tamron lenses tend to focus rather slowly as well. Again for many applications this isn't a problem at all but for sports...ehhh...it could be. I would have to say that some of this too would really depend on what kind of "sports" you are shooting. For example if you're shooting something like a baseball game where the activity is going to be somewhat predictable, so you can "pre-focus" on an area (like the batter getting ready to hit the ball) , then this isn't really a big deal. On the other hand if you're shooting something like football or soccer where the motion is harder to predict, both of these factors could be an issue. From your second post there I get the impression your shooting horse back riding events so I would think as long as they're outdoors, you should be fine. Inside in an arena though might be a problem.
Now something I would like to mention is that if you haven't already taken a look at it, Tamron has a new lens out...it's a 28-300 image stabilized
lens (they call it "VC" for Vibration Compensation). On the wide end, at f/3.5 it's a little faster than most of their other lenses but on the long end, it's actually a little slower at f/6.3. Again for shooting sports specifically, I'm not really sure how much of an issue this would be for you and I'm sure it would depend on the individual and specific shooting conditions anyways. However being a VC since you said you shoot mostly handheld, I think this could be a huge plus for you and it could very well make up for being a little slower on the long end. From the reviews I've read, like all Tamron lenses, this sucker does focus a little slow too but as far as sharpness is concerned, it's getting VERY high marks. I know you said you don't really need the extra focal length but to me, the extra $150 would be well worth it for the vibration control.
To sum this all up, again for the price range, Tamron lenses are really
hard to beat when it comes to sharp images. If image sharpness is your primary concern and you can live with a lens that is a little slow to focus, you can't go wrong with Tamron. It's really a price thing...as I've said elsewhere, most of the gripes I've seen about Tamron come from people who were expecting it to perform like a $1500 Canon L lens...you're not going to get that from a $400 lens regardless of brand. I'm sure if Tamron made an f/2.8 image stabilized lens that focused as fast as a Canon L series or a Nikkor, that it would probably be in the $1000 range as well. If you can live with the performance of that Quantaray lens as far as speed and such goes, I think you'd be very pleased with either the Tamron 18-200mm or the 28-300 VC. Tamron really kicks butt...it just does it a little slowly sometimes
Again just my $.02 worth!