I loved using MPA3 & MAS at Shane's World; it took me maybe an hour of poking around in the programs to learn how to use them, and their support has always been nothing short of spectacular. The only issue I had with either system is that there are some pages that contain typos (can you say "OCD"? I knew you could!). With that said, I hereby offer my proofreading services to Mansion.
Never having used NATS or Carma, I've nothing to say one way or the other about their programs.