I use the BG-E6 with my newly-acquired 5D Mk.II.
I got it new for CAD$185 from a Canon dealer in Toronto. I would not have got it anytime soon had it been full MSRP.
I like it because, being 6'5", it improves the ergonomics. Holding it by the grip is perfect. Having the duplicated shutter (and other controls) is very helpful and adds more control than I thought it would.
Plus the extra battery means that I don't have to charge it for at least a week taking 600+ shots... I still have about 60% life in each battery.