Snowyowl,
I had a Tamron 200-500 for about a year. It was an adequate lens and if you are not after perfection then it is probably good enough. I bought a 300 f4 (nikon and then changed all the kit to Canon - another story) and found the quality was in a different league. I miss the extra reach but I find that the 300 sharpness is so good even with a 1.4 converter that I can crop and still get better results than with the 500 (most of the time). But it depends on your budget and what you are after. Don't get me wrong for the money the Tamron is good but was not good enough for me. If you go to
www.michaelhogan.ie and click on Galleries and then Portrait of a Fulmar, all the shots except the last were taken with a Nikon D80 + a Tamron 200-500 and I am quite happy with them. I have just got the Canon kit and have not had a chance to put up many photos.
If you are not going to take photos because you need a long lens and can only afford the Tamron, then get a Tamron - it is better to be out and photographing than wishing you had better lens and sitting at home