I have the predecessor the 35-350. The 28-300 lens is heavier and larger.
As someone with it attached to a 1Ds, I can wholly state that the weight factor is not to be taken lightly.
The overall picture quality is superb on the 35-350, so I can only assume that its progeny is probably much better.
I am shooting an 11 Mp FF in the 1Ds, and except for the age related issues that comes with, yep age, I would probably say that if you have the ability to get one and think you can handle the weight, go for it.
The 28 also has IS, the 35 does not.
Some main points however.
You must support the lens when using it, and if you let it stress the camera body, you can get lens-body errors. "Error - 01".
The typical advise is that if your not going to use it for a while, detach it from the camera and store it on end,