Most double din are fixed unit's. Because of the way they are fitted it means that they are more difficult to steal when compared to a standard single din. For example a single din can be removed in less that 30 seconds while a double din will take 5-10 minutes to undo the dash and undo all the bolts holding the unit in place. Although a face-off is a good way of deterring thieves it will not stop them breaking in the off chance that the face has been left in the glovebox. It happened to a customer recently and they totally destroyed the dash in his mr2 even though the face wasn't there !!
On the subject of quality, double-din's are of no lesser quality that single din's. Just like SD's (single din's) there is a good range available, from the cheap £200 units to the £2000+ unit's with burr-brown chip's, and touchscreen tft. Unit's like the Kenwood 8029 and Alpine w202 offer excellent quality audio in the larger double-din format. The larger display also makes controlling an ipod much easier. With more car manufacturers now opting to go for this size of unit there will be a greater range of units available in the next few years.