I use this lens as well with a Canon 5D MKI. On a full body camera at 400mm there's a moderate amount of vignetting when shooting a subject with only the sky as a background. On your crop frame body it should be pretty minimal.
Some call this lens a "dust pump" because of it's push/pull design and lack of weather sealing. I've noticed using the lens that this MIGHT be true, my sensor does pick up a fair amount of dust but, the 5D is also known to do this on it's own so *shrug*.
Image stabilizer mode 2 works great for this "sport" it's pretty good at determining how you're tracking the subject and greatly reduces slight motions on the axis perpendicular to the axis you're tracking along.
Also, because of it's size it's a bit of a handful in windy conditions if the wind is blowing perpendicular to the lens.
Another gotcha I found.... Even if you have auto focus turned on the focusing ring isn't disabled and so you can still adjust the focus. I found out the hard way that you can guarantee getting soft images out of the lens if you grip it at the focus ring.
So, just don't grip it there and you'll be fine.
But, despite all that I LOVE LOVE LOVE LOVE this lens for spotting. It's very sharp especially at F8-11. I generally shoot at F9
and I get some really really sharp images. Once you get used to the push/pull and set the tension just right for you the zooming becomes second nature.
Anyways, that's my feedback, hopefully you find it useful.