QF's 'receipt' slip takes about 4 seconds to print - the two occasions I have used VA mobile boarding pass it took around 20 seconds for the agent to look up my record on the system and print the boarding pass. Much slower. It sounds a bit trivial but when the regular boarding queue is flowing freely and the priority queue has ground to a halt behind you people get a bit antsy!
In the future if I have a mobile pass I will ask at the Lounge for a hard copy before I approach the gate.
You can scan your mobile barcode using the virgin kiosks to get a printed out copy of the boarding pass, so you don't have to do that at the gate.