There were two basic options for sleeves that actually fit standard playing-card size cards (rather than the somewhat larger sleeves designed for the Magic: the Gathering card size). The Mayday Games version is 40 microns; the Fantasy Flight Games version is 100 microns. Normally, I'd prefer the heavier one, except for games with a huge number of cards (the deck gets impractically thick, and of course the thicker and more durable sleeves are priced accordingly). There was one catch, though -- for some reason, the Fantasy Flight sleeves are actually a couple millimeters larger than the nominal size. It's not a critical problem, but it's an annoying one, because it's the one thing that demotes a near-perfect product to a merely-acceptable one.
I'll use up the FFG ones I got for testing purposes, but I don't think I'll get any more unless they decide to make them in the proper size.