I have just bought the Tamron and I'm loving it. I haven't had much of a chance yet but I'll be showing some pics in the next few days. The reviews I've read so far show the Tamron is good throughout the full zoom and likewise centre to edge.

I think you will miss that extra 50mm. The bird togs here usually state that 300mm is the minimum effective distance. Although there are situations where that doesn't matter - see http://www.ausphotography.net.au/for...p?71701-Wisdom for a bird shot taken at 60mm.

There is a big difference in price, with the Canon being considerably cheaper. Certainly the Tamron will be better in length and probably image quality (judging from the reviews - I don't have the Canon lens). You have to decide whether that justifies the extra cost. It certainly did for me.