I think they're just paralleled gel cell batteries in a nice case with a couple of outlets, switches and stuff. Makes replacing the innards cheap, but variations in manufacture would dictate replacing all of the internal batteries at once so that the batteries charge up at the same rate.
It makes for an expensive proposition, but they're leak-resistant, portable and yes I have considered them myself.
I opted instead for a single portable power pack with "1200 amp jump start capacity" for my portable power and fixed internal batteries for my long term supply.