The other day I got up to a roof to do some headshots. As you can see, the 50 f1.8 does a nice job blurring the background. The optics are quite sharp too. My only concern is that it does occasionally "hunt" for focus. It's wasn't severe enough to make me miss any shots, but in fairness, it will not focus as fast as an "L" series lens. Nevertheless, I would definitely recommend this lens to anyone who shoots portraits.
Camera Settings: Canon 40D, Canon 50mm at F1.8, 1/5000, ISO 400