I should say I've got nothing against relays or switches, but I've usually found electronics like transistors in an H-bridge to be more reliable and easier to use than electromechanical devices like relays and switches. That is, unless human intervention is wanted, as it is not here.
At a certain point, it seems - designing an h-bridge to handle high-currents becomes something of an art or an extreme science, or a combination of both. For someone who isn't doing this constantly (and thus building up experience and knowledge), it can quickly become expensive. I'm not sure what that point is, but it is likely somewhere between 5-10 amps; beyond that, unless you have a lot of money and time to burn, it is best to leave it to the "experts", and purchase your h-bridge instead of trying to build one from discrete components. I also recommend the use of h-bridge ICs instead of a discrete component layout for lower power designs, unless cost is a factor (or you have the parts and the time).
Then again - yesterday I found a Roboteq AX2250 for $25.00 (in unknown condition - but it looks OK) - I went to their site for docs and such, and found that new, that controller is anything -but- inexpensive. That might make one consider building their own (then again, its a 120 amp controller). They're local, though - so I am hoping to be able to get some help in finding out whether it still works, or if I simply purchased $25.00 of components (maybe the MOSFETs can be salvaged?).