Review based on my experience from the airport to Monte Carlo on a Saturday evening, and back to the airport on an early Tuesday morning.
Airport to Monte Carlo:
Walking outside the airport, it's one of the first booths to your right (there's a sign). $9 one way or $15 round trip to get to the Strip, a little more for downtown. The cashier gives you a little booklet that's your ticket both ways. You go to your assigned number across the street and wait for your shuttle. The driver was friendly, but didn't seem the most organized. He asked us all what hotels we were going to twice. Talked about how we were supposed to get tokens because his scanner wasn't working. He picked up about half a dozen of us, then drove around the airport to pick up more people (not sure if it was the same pick up spot or not). First drop off was MGM and was hopeful that Monte Carlo would follow closely since it's practically across the street...but it was second to last. Driver doesn't tell you in what order he was dropping us off, so it's like a guessing game. It took about an hour to get us from the the airport to the hotel.
Monte Carlo to airport:
Booklet says to call 24 hours in advance. Pick up from Monte Carlo is every hour on the hour...near the registration, follow exit for bus/shuttle/taxis. This trip was much quicker and more organized. Shuttle arrived a few minutes early and driver announced the stops he was making before the airport (only two on my trip). Driver scans your booklet. Both stops were no-shows and it looks like they don't wait past your assigned pick up time. Trip was about 15-20 minutes.
FYI: if you are active duty military, your round trip ticket is free! I heard that a cab runs about $30 one way, so this will save you money...but be willing to sacrifice a big chunk of time.