I have for sometime used the eneloop pro 2450 mAh batteries in my speedlights. They recycle pretty well and have a pretty good run time. There are two things I am very mindful of with rechargeables. The first is to have a really good charger that can assess the batteries and intelligently work out how the batteries show be charged on a battery by battery case. Having good batteries that also coast a bit will soon be killed off well inside the life expectancy is just using a $20 no name brand that just whacks the battery at full charge and continues to charge even when fully loaded.
The second thing is I rarely use High speed sync unless absolutley needed as this hits the batteries and recycle time pretty hard.

Just a word of warning - I used to carry a pack of energizers in my bag in case I got caught out with the rechargeable - I was a second shooter at a wedding and hammered the flash for a bit, eneloops where dead so I used the energisers. When I felt the flash on the side it was quite warm so opened the battery case. Batteries were so hot I could hardly hold them, never had that with the eneloops. Hence I have bought 3 x the batteries I need to carry in the kit now ;-)