I have the 28-75 f2.8 and i love it. I would suspect that the 28-300 won't be as sharp due to the zoom range nor as fast. But it should work well. Tamron does license the mount for use in their lenses.
The only complaints i have with mine is the focus speed. Other than that sharpness only lacks for the lack of a tripod. Be aware that this lens is probly going to be heavy. But the added durability will likely make up for that.
Looks like i have to eat my words about weight it actually weighs less than my shorter zoom.:?: look at http://www.pbase.com/cameras/tamron
for shots taken with it.