I've been using the Hobbico Air Alert on my planes for a couple of years now. They seem to work OK. The one thing I did do was hook it into the aileron channel with a Y cord instead of into the throttle channel. If I'm doing lazy figure eights or doing slow circuits waiting to land, it can go into "lost airplane" mode as I can fly around for a minute or more without touching the throttle... I can't however fly very long without using the ailerons!

Edit: Do a search on Tower Hobbies for "Hobbico Air Alert" and you're set!