I've been doing the South Beach diet for about 3 months now, which is one variation on the one million low-carb diets out there. I've lost between 14-18 pounds, and have never been down less than 10 pounds since the first two weeks of the diet.
I think I'm kind of an unusual case because prior to the diet I exercised a lot but had a very poor diet with a lot of junk food. The junk food went away just as I increased my exercise during summer time, and the pounds rolled off. My fiance's participation together with our joining a Community Supported Agriculture food delivery program also helped a great deal (the program is Eat With the Seasons and I highly recommend it to anyone living in San Francisco or the South Bay).
My sport activity is rock-climbing, and I run for exercise. The diet's made a huge difference in my indoor rock-climbing at the gym. It doesn't feel like it's made that much of a difference outdoors, where my mediocre climbing is limited more by technique and psychology than by strength. The diet at first made almost no difference in my running during the initial, no-carb period. Once I started adding some carbs back, I started improving, and ran my first half-marathon yesterday.
An accurate description I've heard is that you don't feel hungry on the diet, but you do feel deprived. It's interesting that I don't crave light carbs, but the heavy, whole grain stuff.
The down side is that I eat a lot of eggs and a lot of meat. I'm curious as to how it's affected my cholesterol level but haven't been tested.
All in all, I'd recommend it, at least for someone who has a bad diet to begin with.