This is a rather tough question....
The mk II is an amazing lens in every way but one. It has somewhat harsh bokeh, especially compared to the earlier versions.
Man is it sharp though, its tough to beat it in this area. The color/contrast are also out of this world.
It is very likely the best zoom lens ever made. I welcome different opinions on this though (really on anything for that matter).
I would say that it is not worth the updrade over the IS mk I for most people.
Hope this helps.
Edit: My 70-200 2.8 mk II is the first lens in this range that I have ever owned, so I have no frame of reference.