I put an RCD510 in my '06 GTI back in June, having only bought the car in May. Everything was fine for the first week, having driven the car every day. Then, after having not driven the car on the saturday, I went out to start it on the sunday and the battery was completely dead.
I've since bought an AD version Canbus, but have not been able to find anyone to fit it and code it for me. I still use the RCD510 every day in the car, but take the radio fuse (F19) out every night to stop it draining the battery.