Canon surprised a few people when they launched this lens in 2009. Let's face it, there weren't many people complaining that the original 24mm f/1.4 L was a bad lens. But the high resolution sensors in modern DSLRs place very high demands on the lenses, and presumably Canon thought the Mk I version, which dates from 1997, didn't quite cut the mustard in that area.
This lens is bigger and heavier - and noticeably more expensive! - than the Mk I version. Tests show that it's sharper wide open, a bit sharper in the corners of a full-frame image, and a bit more resistant to flare. If those things are very important to you, then this is the lens to go for. But for most of us, the Mk I version is still a very decent lens.