Never used the Tamron but have the Sigma (Stabilized version) as my main walkabout lens.
IQ wise, it's a superzoom, so don't expect it to be pin sharp, but unless you go pixel peeping it's unlikely you'll have any complaints. Has never stopped me doing a significant crop.
Focusing is slightly loud, but fast enough especially in decent light, although not as fast as the Canon 18-200mm.
Aberration has only really been apparent when shooting extreme contrast subjects (like a leaf contrasting against a completely clear sky) but this is so easily cured in post (Photoshop or Lightroom) that it's a non issue for me.
Bokeh quality, not going to make you say wow, but not going to leave you disappointed either.
Overall, would I buy this lens again? Most likely, the only thing that would possibly stop me is something like a very good deal on a 24-70/2.8, but then again having all the way from 18 to 200mm in a single package is very nice, and for the price I'd definitely say it's good value. A free hood that Sigma include is a nice bonus too!
Most shots (done in 2011) on
my Flickr are done with the Sigma, if in doubt the EXIF should tell, or just ask.
If there's any tests you'd like done, I'll do my best to help out.