Maria - The autofocus on my Tamron is extremely slow, and will hunt back and forth often while trying to lock in on common targets. In order of the budget superzooms I'd recommend: Sigma 150-500 OS, Sigma 50-500, and the Tamron last.
Here's the photostream of one of my contacts who uses the Bigma (50-500mm)
http://www.flickr.com/photos/hvanneyri/
I would advise that he seems to use expert technique and rarely shoots in subpar lighting. The typical user won't be able to squeeze out the same IQ.
Here is one of my favorite shots I took with the Tamron 200-500, but as I said I think it's an inferior lens. Several people on this forum advocate it, but I have never seen consistent output that can rival the Sigmas.
http://www.flickr.com/photos/davidra...7600985744723/
Specifically, center sharpness, contrast, and colors are worse. Get a Sigma and go out to kick some butt!