Shimano has taken their venerable SPD pedal and encased it with a tough composite cage to give you a little more room to work with when you've got to click out for those hair ball technical sections. The PD-M647 is double sided and features easily adjustable release tension. The axles are heavy duty cro-mo and the pedal spins on high quality cartridge bearings to ensure long life.
These are great pedals! I say all-use because I have even just ridden them like a platform for short distances because the clip doesn't protrude too much. They are light enough for fast singletrack rides and tough enough for AM and some DH. This extra support around the clip also comes in handy if you want a little more foot support on a DH or AM type ride. They are very durable; I have cracked them on a few rocks and since the black material is more of a hard rubber than a plastic they don't crack and fall apart. They are very adjustable. You can easily adjust each SPD clip with just an Allen wrench and they are well marked for which way to turn for these adjustments. I like to ride mine as loose as they will go. It still provides plenty of retention to keep my feet on the pedals, but they also easily pop out if I need to bail or if I wreck. I have yet to wreck and still be clipped in while falling. That provides some peace of mind for those that are scared to clip in for fear of endless tumbles wrapped up in your MTB.
I bought these for a second DH bike I ride XC/FR around my area with. So pedaling up hill with a 48lbs bike can be tough but a great work out. I find using clip in's on the uphills helps me climb easier and accelerate when I do it. I had Shimano PD-M636's that everyone loves and no one knows why Shimano stopped making them but for a replacement, these are awesome! Just as good as my old one's only the 636's I could take apart and service with new grease when I wanted to. Haven't seen whether you can on these but so far, they're incredibly well built, light weight, the composite plastic or whatever the base is, is very durable and unless you're pro racing, would highly recommend. Only reason I don't for DH racing is the serious rocks that may crack the composite in a bad crash. For everyone else, their 5 star man. They clip in so nicely and hold my feet really well into place without any play. I'm all about Sram man, but Shimano really knows how to do pedals well! GREAT buy!