Firefly,
there is a useful review of the MKII here http://www.canonrumors.com/reviews/c...ii-usm-review/

I currently own the MKI so in no real hurry to upgrade, but it appears this is an overall better performing lens so if buying new I would seriously consider it. Only downside I see is it's considerably more expensive and 20g heavier.

Cheers
John