It seems to work all right for me so far. I upgraded to a GDL393D after I experienced a gyro failure in IMC--it made me really appreciate the need for a backup. I have it set up to provide position and attitude to both an Android phone and my iPad Mini 4. I also have a bcakup android tablet in my flight bag, but it is old enough that it can't use the attitude data for synthetic vision.
I would offer the following cautions:
Make sure your Android device is new enough to take advantage of the attitude data.
The attitude readout is not instantaneous. It is probably fine for smooth air with gentle roll and pitch rates but if you were getting tossed around in the soup it probably would always lag a bit behind. If you get one, fly VFR with it and understand how quick (or slow) it is to respond to various situations. I'd recommend some hood time with a safety pilot if you really plan to use it as a backup!
It seems like the unit "recenters" itself somewhat frequently. This can be disconcerting if the horizon suddenly changes on you.
It looks like the GDL393D has to be aligned fairly closely with the longitudinal axis for yaw indications to be accurate. I have mine velcroed to the glare shield and it took a bit of alignment to be acceptable, but it seems like it never quite matches my slip indicator.
Understanding these limitations it is probably a decent emergency backup but I wouldn't want to fly on it in the soup for a very long time...
