I've used it - really helpful. Wish they would send their testers eye tracking monitors - that would rock.
Also, do they have a "more users, fewer minutes" option? I know I'd rather have 15 users for one minute each, or seven for two minutes each. After that, most users will have decided to stay or flee any ways. It would be good to get a sense of that early decision-making, especially across a statistically-reliable sample.