Ive used some, they are expensive and you have to be careful if you want the right colour tempreture, the ones i put in were about 1500lm but cool and great for there job of lighting a small path way, the warmer you get the less light output. The optic's make a big diffrence too.
If your looking at getting them you need to invest as the only ones that are good are expensive. Look out for the cree LED's they are great, altough there are more led's currently coming to the market like Luminus which are kicking out well over 100 lm/w.
Solid state lighting is the way forward!