We tried (rented from Lenses For Hire) the f/4 IS and then the f/2.8 non-IS for a couple of events. Loved the IS but loved the extra aperture stop (shallow DOF is a strong favourite for me!) - from the many comments and even more images, I decided that the f/2.8 (first stage - decision on f/2.8!) non-IS was slightly sharper than the IS. OH preferred the IS
With animals (pets and wildlife) being major subjects and the fact that, despite loving to show motion (but in the right places!), high(ish) shutters would probably be used
So I got the f/2.8 non-IS
OH got the f/2.8 IS
And do you know I never borrow the IS version as I find I can get everything I want without IS (but I still love my 15-85 IS and 150-500 OS!)
Here's one of the first shots I took with it - Bolton Abbey behind