I have installed the recommended sonar sensor (XL-Maxsonar EZ4) on my custom-framed Arducopter. Using the ArudPirateNG code, I've been flying with the Altitude hold and it works great. I love it! It allows me to put the quad at the altitude I want, say five feet off the ground, flip the Altitude hold switch on my Tx, and then fly around at will at that altitude. This allows me to practice my yawing, pitching, and rolling without having to worry about crashing into the ground. (Still have to watch out for trees and buildings!). It's very cool. Thank you to the team!
I'm really looking forward to automatic take off and landing in the future, and also obstacle avoidance (using yet to be installed side-firing sensors obviously). I have included some pictures of how I mounted the sonar sensor underneath my quad. I have also included some pictures of the tiny EyeCam camera (made by DraganFly Innovations) that I installed on my arducopter. It transmits video in real-time back to a 9" portable screen and a small video receiver that I rigged up with a battery.