Vernon: I've got the MK2 and it's by far the best wide I've used on the Canon - corner to corner sharpness /contrast has improved; they've also worked on the distortion; lastly it's a very usable lens at f2.8...I never shot anything wider than a f3.5-f4 on the mk I, but nowadays, it's f2.8 90% of the time.
Only downside is getting a 82mm filter, esp a slim cir-polariser and B+W UV MRC is going to cost you a bit.