I'm a commuter, weekend rider, and occasional competitor, and I use both my road and mountain bikes for all of these on a regular basis. I chose these pedals for their reported comfort with both cleated and flat shoes, and I chose the PD-A530 on my road bike for the same reason. I only have one pair of cycling shoes, so I wanted the same cleats on both, and all the pedals had to be comfortable with flats.
Although I love the PD-A530's, I'm really disappointed with the PD-M647. I forked up big money on these nice pedals for my new titanium XC hardtail, thinking they need to be good with flats and light weight to match the rest of the bike, so these are my only option by Shimano. As it turns out, they aren't really much more comfortable than normal cage-less SPDs, which makes me wonder if I should have even gone with these at all. Secondly, there's an obnoxious creeking sound coming on every pedal stroke, which drives me totally nuts (reported by another reviewer also), and although the cage hasn't cracked, some other plastic part on the spindle has cracked on both pedals. All these combined, I ask myself why I keep these every time I go on a ride. For a similar amount of comfort, I could have paid half the price and saved half the weight with a pair of Crank Bro's Candies or even Shimano cage-less pedals, and been far more satisfied with the end result.
I guess these pedals do have two things going for them: 1. If you miss the clip you have a bit of platform to pedal with until you get a chance to clip in. 2. They apparently make good, lightweight downhill pedals (but I don't downhill, so thats no help for me).