I was carrying some Duracell rechargeables on the weekend when Kory, Matt, Sidd and myself met up for a shoot, and they failed in my flashes, even though they'd previously been fully charged using a Powerex charger bought from here. I had to borrow Kory's Eneloops and use those (thanks again, by the way) - unfortunately I'd left my Eneloops at home... Oh well, at least it worked out in the end.
The Eneloops are awesome, by comparison, so I'll be buying another 8 more of them soon so I always have them ready to go on hand even when carrying two flashes around like I had on the weekend.