I fitted both front and rear. I first fitted them onto the car with my standard S4 suspension which had covered about 80k.
They certainly reduced the body roll, it goes nice and flat round the corners now, but the difference wasn't as life changing as a lot of owners seem to suggest.
That's the problem with these forums, I think people read how amazing something is, then as soon as it's fitted it's "totally transformed" their car. I think the placebo effect has a lot to do with some things!
If your standard shocks are in good nick, I would recommend ARBs over coilovers simply because I think the benefit/£'s was more for the ARBs than coilovers.
I only went for coilovers because I had a leaking damper. I actually preferred the ride of the standard S4 setup with the H&R arbs.