Short summary - awesome, awesome experience, don't wait, make your reservation and do it!
Long summary - my wife and I have done the night snorkel in both "location one" (south of Kona near the hotels) and "location two" (up close to the airport).
Manta Adventures takes you to Location Two, and it's amazing. There are limited mooring spots, so the way MA handles this is to leave the dock early, well before sunset. This not only guarantees a mooring spot, but you get to spend some time on the boat socializing and watching the sun start to set. After mooring we saw a ton of trigger fish from the boat, and even a monk seal working its way along the coast.
The mantas are of course the main attraction and it doesn't disappoint! We were in the water with them for at least half an hour (I lost track of time) and had up to three mantas doing barrel rolls under our light board at once - close enough to touch (please don't touch them, but don't be surprised if you get grazed by a wing as they roll by) and simply awe-inspiring.
The captain and crew were very friendly and informative, and did a great job of working through everyone on the boat and making sure they were comfortable, the snorkel equipment was sized/fitted correctly and that everyone was enjoying the trip.
Would definitely repeat the experience with Manta Adventures!