It's much, much easier getting in and out of a clipless pedal than toe clips...

The only minor problem is if you're doing a steep downhill (off road!) with corners and stuff it's more difficult to weight the outside of your pedal for extra traction, and more hassle to put a handy motocross foot down.

But for cross country and road you can't beat them.. Best are Crank Bros, as has been said before..