I'll say go if 70-200 F4 on-IS is still expensive and you need 300mm instead of 200mm at the long end.
Tamron is very nice lens, but not the L lens by IQ.
VC is also a big selling point for this lens. Shooting @ 200mm on a crop without stabilization will require at least 1/320s shutter speed. It is sometimes difficult to get that shutter speed, esp. of your camera sensor doesn't have good high-ISO capabilities.