I had the BMW tank bag last year, but sold it so I could afford to buy the ali top box. I've just come back from a 3100 mile trip to Austria, and really missed it when fiddling about at tunnel terminals, toll booths etc. - it's a handy place for change, tickets, somewhere to put your gloves etc. I think the BMW bag IS expensive, but then again, what BM gear isn't, and it's certainly easy to take on & off the bike.
Regards,
Charles