been looking at these myself for event photography
you can get a lithium ion BEAST for £300 ish
or £100 for a canon pack
However, the cheapy ones are about £35 on ebleh.
for power?? well the aren't anything special, it's like a grip battery pack.
They use normal AA batteries. However, with some lithium AA's in there or some decent brand 2500mAh (or I think I have some 3000mAh ish ones) then the 8 batteries or so in parallel will give you obviously longer charge. But the current you get will be more even I think and will charge the flash unit quicker. It could be that the pack runs 2x4 in series/parallel which would give you more current (faster charging)
I'm going for the cheapy version. The canon version will be better water proofed I think and superior build quality