I would recommend KEH without hesitation. I've used them for years without problems. You can probably find better deals on ebay but its always a gamble on what you get. Also KEH's bargain rating, as long as its says the glass is clean and defect free, usually is a great deal.
DaveN is absolutely correct - their rating system (on condition of equipment) is very conservative and it is usually better than they grade. I have bought (and sold) with them several times. Last time I bought was a 1D4 body and it was rated bargain but the phone rep told me they rate low and it would be the equivalent of good at someplace like Adorama. He was correct and in fact I would rate the body I received as good to excellent.
They have the largest supply of used equipment of any USA dealer and accurate descriptions and condition will be better than listed. There was only one time I had an issue with them and they accepted return no problem. That was a body I ordered that reeked of cigarette smoke as the former owner was obviously a chain smoker. They should have listed that in the description (which they did not), but as I said they took it back and refunded me no problem.
Thanks, all! I've wanted a MP-E for quite a while, but because all of my bad service experiences with Canon, I've decided to wait 'til I've had at least one good experience before further investing in Canon.
My experience was not as good. I ordered a Canon 24-105 over Black Friday rated in EX condition. Everything I read basically matched what was said already in this thread, so I figured I would be OK and maybe even pleasantly surprised. (And it was a good deal with the discount - same as today.) At the very least, I thought the glass would be clean and 100% working order. The lens I got however, the grip on the zoom ring was loose (so you had to squeeze it really hard to zoom or it would slip), it was pretty dirty, and had dust/lint all over it. It was probably just an unlucky first experience with them but I opted for my money back instead of an exchange. Not sure if I would buy from them again?
Having said that, their customer service was easy to work with and setting up the return was no problem.