They are a technology that seems to be more-or-less mature, but I hadn't seen them for sale for domestic use up to now. Not sure about their cranking current capability for starting the diesel though! I'm hoping prices will drop in the next few years and an electric motor will become a serious possibility. Have you seen the prices at the moment? (here) . A 100W stack would be worth more than my boat!
I reckon there's another few years needed before this becomes affordable for most folks. Even then you'll probably still need your batteries to smooth out the peaks and troughs of your power consumption, these fuel cells are more likely to substitute for your generator than your batteries.
Definately worth keepig a watch out for how the technology develops though!
I was thinking of something that would drive my boat without the need for a noisy diesel engine, or even a quiet one, but at 50 watts each I'd need at least 20 of these things, which is about US$200k. It would be cheaper to buy a long mains lead!