I have one of these 'lean, mean grilling machines and I think it's great. Cook food nice and evenly, very obviously cuts down on the fat intake as you can actually see the fat running off of your meat and incredible easy to clean. I wouldn't be without it.
They are good as they do drain out all the juices as has been said but they also seal in the moisture and cook quicker so a chicken breast is much more juicier than one done in the oven and you can do toasted sandwiches in them too .The drawbacks are they are awkward to clean and the non stick lining on mine is starting to come off now but they are only about �15 for a new one so it won't break the bank .
As a non-cook, I couldn't live without my George Formby. (Although I am biased as my dad works for the company) You can't burn food on it, it cooks twice as quickly than on the grill, you can do veggies on it, kebabs and fab toasties. It also keeps my other half's steak seperate from my veggie option. I think the lean mean bit is a bit of a marketing swizz as the fat drains off on a grillpan but my dad would porbably contest that!
They're good, especially at crisping things up. But I'm not entirely convinced that they are any healthier than a normal grill - cooking sausages on a normal grill all the fat just drips straight down and away. The angle of the GF grill means that it runs away far slower.