I scheduled my ME add-on about 3 weeks ahead of time. I wanted to do mine during a certain week, and they were able to accommodate me no problem. I went through with one other guy, and I though that was pretty helpful. We were able to help each other out in the sim.
My program was: Day 1= ground and sim, Day 2= sim and flight, Day 3= sim and flight, Day 4= oral exam, we got weathered out for the flight, and did the flight on Day 5.
The sim is a Frasca, the cockpit representation is pretty good. The "outside" is something like a 27" TV running MS flight sim 95. Not great visually, but it's not for sight seeing, it's for running though procedures.
I had about 320hrs going into the program, and I think it's good for what it is. It will get you a multi-com. You won't be a great multi engine pilot at the end of it, but you'll be able to do the checkride pretty well. You may want to build in a bit of flexibility into your travel plans, if there's been a string of bad weather, they'll be playing catch-up with their career students, and although the multi-com folks get some priority since we're not there for very long, between scheduling planes, instructors, and DPE's things can get a little tight.
I liked it enough to do the program this April in JAX.