I had a Mk I for about three years and was never really impressed. It seemed to be a little less sharp than either of the two non-IS models I'd owned previously. However, the biggest problem was light falloff throughout the zoom range, not just at the short end. I sent it back to Canon three times for a fix and and it never seemed to get better.
I can't say I had much luck with teleconverters either. I would occasionally use it with a 1.4x EF II when I didn't want to carry my 300 and I would almost always regret my choice. And, the one time I matched it with 2x EF (II), it was such a bad fit that I don't think I got a single sharp shot.
In the last 20+ years, I've had nine Canon zooms in this range of focal lengths, starting with the 80-200/2.8. The IS Mk I was definitely the worst. Maybe I just had a bad copy, but I'm not the only one, as I've talked with others who had similar issues.
The newer version seems better on all counts, greater sharpness, especially wide open, particularly at the long and short ends. There's still some light falloff in the corners, but it's noticeably less. And, it gives reasonably sharp results with my converters, especially the 2x EF III. The images are still no match for what I take with my 300, but they're usable for most everything my clients require. In my book, the Mk II is well worth the premium over the earlier model.