I am a roadie, but I regularly ride with MTN bike shoes (Sidi Bullet - which I highy recommend), and Crank Bros Candy SL pedals. I have never ridden with any other pedal system, because I am too happy with what I have! VERY easy to use, and I have not fallen in two years - at least not because of the pedals! They are comfortable (no hot spots) and plenty of float to prevent knee pain. The Candy line of pedals can be bought for ~ $70. for Candy C, and ~ $120. for Candy SL (if you shop around). I find that my Candy C's squeak a bit, so I put them on my MTN bike and spent the extra $ for SL's for my road bikes.
Upsides:
o You can click in on either side of the platform, and the integrated Egg Beater mechanism gives you four ways to get in...
o You can get out toot-sweet!
o They have a nice stable platform
o They are simple and clean
Downside:
o Not super easy to find the cleat-clip interface -- especially when you need to get the lead out... i.e. a truck is coming and you are dorking around with your feet!!!!
o Do not always let you know you are in -- or worse, not in... Sometimes the click is so subtle it's inaudible! After a brief period, zero problems...
o The Candy C's can squeak (I hate squeaks!)
o They can be slick (sliding off the pedal is not cool)
o With some shoes they can fit tight - i.e. preventing the float you need, so make sure you expriment with the shims if you feel your feet are not moving freely (laterally)
BTW, I believe MTN bike shoes are not as stiff as road shoes... I like this, because I like to walk around in mine. If they are not as stiff as a road shoe, you will lose power as a result of the flex. I think the Egg Beater pedals can be fitted to a road shoe, though I have seen that Crank Bros now makes a road-specific variant.
My grade 1-to-5 scale... 4.5