I used Shimano SPD cleats and pedals for several years without any problems. However, last year I started spending a lot more time riding in the mud and needed a pedal/cleat system that could handle these conditions better, so I switched all three of my mountain bikes over to Crank Brothers Eggbeater 1 Mountain Bike Pedals.
The open-body design of Crank Brothers Eggbeater pedals shed mud and snow like nothing else, so if you love muddy singletrack you will probably love these pedals as well. The pedals have a wide adjustable release angle (15 to 20 degrees) and a forged chromoly spindle with cartridge bearing seals to keep the dirt out. Because your cleats can engage the pedals on all four sides you never have to look down before clipping in. A pair of these pedals weight just 256 grams.
While these pedals shed snow, they are not so great when it comes to shedding ice. If you have to get off your bike and walk in slush for very long there is a good chance your cleats are going to clog up with solid ice, and I’ve found that very difficult to get rid of. So, if you are going to have to ride much in slush you might be better off with something like the Odyssey JCPC Pedal (no cleats).
Crank Brothers Eggbeater 1 Mountain Bike Pedals retail for $60 a pair (including brass cleats) and come with a two-year warranty. If you have several mountain bikes you would like to change over to these pedals, talk with your local bike shop about buying them all at the same time—you will probably get a better price.
If you are a Weight Weenie you might be interested in the insanely lightweight Crank Brothers Eggbeater 11 pedals. The four main parts of this pedal are made of titanium and they only weigh 174 grams per pair. The downside is that they cost $425 a pair (I don’t plan on purchasing these anytime soon).