I've used clipless pedals for years and find them far superior to flats for my type of riding. (I ride predominantly XC or road) My more hardcore DH/Freeride cycling buddies have changed back to studded flats and shin protectors). The only real 'moments' I've had were with traditional toe-clips. Oh and I did very nearly crash a demo bike outside the shop because I had Specialized cleats and the dem bike had Shimano SPDs. They go in ok but don't release. D'oh!
I use the Crank Bros Candys on all my bikes, which means both mountain bikes and the carbon road bike.
To me, they have these advantages:
1 - clip in from either side without having to spin the pedal round (unlike most road cleats)
2 - one pair of shoes for all my cycling (and therefore space and £££ saved)
3 - plenty of float to preserve my dodgy knees
4 - off-road I'm less likely to clout my shins like the guy on the previous page (I have done before and know how much it hurts)
5 - greater familiarity with clipping in and out because it's the same on all my bikes
6 - mountain bike clipless cleats are small and are recessed in the shoe so you can walk without getting tipped back, scratching cafe floors etc etc.
7 - compared with flat pedals they're far more efficient
Admittedly they're probably not quite as efficient as a pair of pure road pedals, but for me the other advantages outweigh that factor.
Also the Crank Bros pedals do not have adjustable cleat tension. This has not generally been a problem but I do find that they tend to release rather easily if you twist your foot, say, to look behind you when stationary. I get one or two odd looks or smartarse comments from the occasional 'purist' roadie, but I just tell them it ain't a fashion parade and I'm not racing anyone. If I only had a road bike I'd consider pure road cleats/pedals though.