I owned the mkI for about a year. I shot it on a 40D primarily until I got the 5DII then I found it too wide for my liking. So I sold it.
I bought the mkII a couple years later thinking I needed a fast wide again, but found it still too wide. So I returned it after a week or two.
Now to answer the question, the mkI, I found to be soft wide open but reasonably sharp by f/1.8 and higher it was fairly sharp. The mkII in the short time I had it, I thought was nice lens. The colors were definitely better than the mkI. I found the AF to be slightly faster and more accurate. It definitely didn't search as much. It was definitely sharper and more contrasty wide open. The corners were also notably sharper. I'd almost liken the mkI IQ to that of the 50 1.4 where as the mkII is almost in the ballpark with 50 1.2, 85 1.2II.
If you shoot wide open a lot or need the weather sealing I'd say go with mkII. If you'll be shooting stopped down a bit and don't need weather sealed the mkI is still a decent lens and you could save a couple bucks.