Is this teleconverter any good at all?

I think you need this version of the Tamron.

http://www.bhphotovideo.com/bnh/con...mp;A=details&Q=&sku=185152&is=REG

 

I can HIGHLY recommend this converter and vendor:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=3343&item=7523200049&rd=1

Regards
JohnG

 

I have a Kenko Teleplus like John G recommended, which I use with my 180mm f/2.8. It does not soften the image at all, but does lower the contrast. No problem, I add a bit in post.

 

This is the version that I recently purchased. It will work with both AFS and non AFS lenses. I haven't done critical testing but cannot see a degradation in image quality with my 300 f4. It does slow down focusing a bit, but I was still able to do birds in flight with it attached, handheld. Haven't tried it yet on my 80-200 afd.