I have both a Tamron 180mm and a Sigma 105mm, I bought the Sigma first but it doesn't get used much since I bought the Tamron. It's a great lens and the extra reach is well worth having. I also have a Kenko Pro 300 2x teleconverter and that combination is really useable as long as you have bright light or good flash.
This was taken with the 180mm alone.
This one was taken with the 180mm and the 2x as the rock it's on was in a lake and I needed the extra reach.
There are many more samples on my site http://www.pendery.info