Got any reference on the efficiency and longevity of their fuel cells?

Hydrogen fuel cells have a sweat spot at around 30% efficiency lasting for several years, and another at around 50% efficiency lasting for a few months. I have never seen anybody get a different set of numbers.

The Toyota hydrogen fuel-cell car has a high price and not so great performance, so it's safe to say that the answer to your question is "not good enough for a car, yet".

No idea. 30% is competitive with an internal combustion engine though.

(and for a heated house you'd get better effective efficiency because the waste heat can be utilized)

They are competing with batteries, not combustion engines. That's because the hydrogen must be generated somehow (and stored).

If you go and generate the hydrogen from natural gas (so you have a natgas powered car, not much gain) you'll have to multiply those by a 40% to 50% conversion efficiency. If you generate it from water and electricity, your equivalent battery will have that 30% efficiency multiplied by another ~30% efficiency of electrolysis.