I've got a mishi in mine, had it for a couple years now, and hasn't leaked on me yet. Although I hardly ever drive that truck and not very far at that. More of a toy these days. Probably less than a thousand miles on it.
To all who may consider one for an idi though, a word of warning. The unit I received REQUIRED the use of a 19psi proprietary cap. On my unit, a regular style would not work properly. I know this has been a hotly debated topic, but when I installed mine, I completely redid the cooling system, and made several mods/changes for the better, and had good reason to believe there was no cavitation issue, so I felt that it wouldn't be just fine for my application. So far I've been right. If someone was not in the same boat, you may start finding the holes in your gaskets pretty quick.
Now the one in my 96 stroker has been abused pretty good, and I've only had to replace it once. And it's questionable weather or not that was actually a workmanship defect. It sprang a pinhole in a tube, in a spot that had not been touched, so it seemed the aluminum was just thin there. The ps trucks use a Degas bottle, so no change in pressures there.
Edit- I would also expect to pay about $500 for your lifetime warranty misimoto. But, it sure does look pretty under the hood