Not sure where the appropriate place is for this sort of thing, but recently I was having trouble with my MB1200 sonar that was shipped with my 3DR kit. No matter what the distance, I always received the same output of 765cm.
I contacted the folks at MaxBotix and they provided great troubleshooting support and recommended I return my sonar for a checkup. I received a return shipping box and included was a new MB1200! I tried out the replacement and it works like a charm!
These days customer service will make or break a business, and MaxBotix seems to have it dialed in. When considering a sonar, I'd recommend MaxBotix products for their great customer service and support for the DIY Drones community.