The 520s are good and I've been using them for a while. Shop around and you can get them cheap to. From my source (www.merlincycles.co.uk) i can get a pair for slightly more than a pair of replacement cleats! It's the economics of madness!!!

There is one thing though. i've had one pair where the seals/bearings were not quite right. I thought the left hand M520 felt a bit suspect before I put it on but I wasn't too sure. Needless to say after about 50 miles the grease had spilled out the crank side and the bearings were dry. i got them replaced but it is worth giving them a wee spin to check the smoothness prior to putting them on your bike

They can be user serviced although you need the Shimano tool to get them apart. It's only few bucks if ordered through your LBS but it means that you can replace the bearings if you need to.

If your into looks the silver ones 'wear' better than the black ones.