Whats the benefit and downfalls to using a Megaohmeter?
It depends on what you want to do with the information. The readings will give you an idea of how good/bad the insulation on the windings actually is. On small systems it may give the customer some extra time on a compressor by giving clues that there might be moisture or acid in a system. There is plenty of information on the "Fluke" website for this type of testing instrument.
There are many different meters that can read megohms, the more voltage that they put on the system, the more thay cost. I don't believe that you need a crazy expensive unit if your main line of work is in residential.
I just have a low cost supco megaohmer about a hundred bucks it works fine they come in handy i had a 50 ton carrier blowing fuses to the condenser fan circuit with multiple fans but it would only blow about once a month insted of guessing which one it was i just checked each circuit with the magaohmer made it easy to find i dont use it often but they do come in handy