I had the mkI for about 10 years, a good copy apparently because I never felt the need to upgrade or swap it out. A friend of mine had the mkII (traded up from mkI) and convinced me it was a worthwhile upgrade. I was also feeling like my mkI was getting long in the tooth, and I'd rather upgrade before it needed any kind of repair, and while I could still sell it a full market value. So when there was a sale on the mkII with rebates, I picked up one.
I tested the two side by side a bit before selling the mkI. My test indicate at f/2.8 in particular, the mkII is significantly sharper, which is what my friend told me to expect. From f/4 and higher, not as much of a difference, but the mkII is slightly sharper. As for the lens hood issue, the mkII has better contrast and flare resistance without any hood than the mkI does with a hood, so the 'superior' hood design of the mkI is completely invalid when comparing the two. The build quality is good in both, but knowing the inherent issues that many have had over time with the mkI (where it easily goes out of 'tune' from heavy handling or even normal usage), there is at least a psychological feeling of confidence in the sturdiness of the mkII that I had lingering doubts about in the mkI, even though my personal experience never validated the concerns for the mkI. One other noticeable improvement with the mkII was the autofocus performance, especially with current camera bodies like the 5DmkIII. Simply put, I get more keepers with the mkII, it seems to get good focus closer to 100% of the time, much more than the mkI ever seemed to. I'm also comfortable shooting at f/2.8 with mkII, while with the mkI I always felt the need to stop down at least 2/3rd stop whenever I could. What good is a 2.8 lens if you never feel comfortable with results at 2.8?
Bottom line, the mkI was a good lens for me, and I was happy with it until I got the mkII. I wasn't happy about spending the money, which is why it took me so long to switch. But after the discounted pricing which I was able to get the mkII for on sale at the time, and what I was able to sell the mkI for--pretty close to full market value since it was an excellent copy of the lens--my cost to upgrade was about $800. I feel that it was worth $800 to upgrade, might have even been worth more, but glad that was all I had to pay.