welcome to the forum
I looked at this lens also for a 5D, I came to the conclusion that it wasn't the Wright lens for me and went for the 24-70. As I now understand with any zoom lens there are compromises and as the focal range increases those problems are compounded and 28-300 is a very long focal range.
Now if you have to have that range in one tough well built lens and ultimate image quality is not the most important factor then this is probably the ultimate choice.
Bear in mind however that the lens weighs an absolute ton.. It costs a fortune and as soon as you have purchased it you will have lost half your money, (this lens depreciates more than most other lenses).
So you need to weigh up the pro's and cons and decide whether this lens suits you..