MkII is definately worth it...but I feel the original non-IS model is just a "touch" sharper. The IS at 200mm or more with the extenders makes up for it though.
Really boils down to if you need the IS at all. Some people use tripods, have steady hands, cant afford the IS. I would not get one with the Mark I IS. They are soft all over the place and was a mistake to release as an L lens. Original and MKII versions were much better.
If you already own the non-IS version, you will get a pretty good trade in for it depending on condition...which was what I did.