My one concern: I'm planning on getting the Canon 70-200 f/2.8L IS II once I can scrape together the cash.
Assuming I have that lens, adding a 2x and 1.4x teleconverter (which themselves cost only a bit less than the Tamron lens) would make it a ~200-560mm f/5.2.
With the teleconverters I could carry less gear to have the range without not carrying what I hope will be a favorite lens (my 70-200 F/4L is my current fav). I'd also have more flexibility (as the tc's can be used on other lenses). OTOH: well, I'm just not sure which way I want to go. I am really excited by the Tamron.