I did my MEI at ATP, and the best advice I can give you is know the booklet they send you on the Piper Seminole front to back. If your still fresh from doing your initial CFI and CFII the rest of the stuff should be easy. My experience was great, only took two days and cost was fair. As far as scheduling I would call 1-800-ALL-ATPS, there are a bunch of instructors there answering the phones that would be more then happy to help you.
I did my initial CFI and CFII at Shebles. They have since raised their prices considerably, $1895 for the MEI. ATP has a special for 895 plus the DE fee, I'm just wondering if people end up having to do more flying at 200/hr to get it done. I know at shebles it was pretty much a fixed price if you had a half a clue.