I don't think it really takes much more fat than any cooking over a grate. Figure on a grill (Foreman or otherwise) you've got lots of room for fat to run off. Since it's nonstick, it's awesome for stuff like fish. It's superior to a skillet since in a skillet you're leaving the food to soak in the fat.
It's a dry cooking method, so your food will be a little drier. It's also not hot enough to sear really well, so you won't have much extra browning (based on my experience cooking on 'em). I prefer a charcoal-cooked burger, but a Foreman with the removable cooking surface is much easier to clean up.
I tried cooking ground round on a Foreman, and it was awful. Dry as the Sahara.