We've got one, but not for camping. We're living in a shed whilst building a house, so have to use it for showers every day.
Anyway the first one buggered up after about a month, it was never easy to light from the start but though it was just our experience - or lack of. Anyway the 2nd system (we exchanged the original for a new one) has worked without a flaw. Yes as the car battery drains and the voltage drops, the pump must go a tad slower so the water is hotter, at least we know when the battery is getting low!
If anyone brought one of these around Dec / Jan (07/08) then I heard from a Camping store that there was a recall on them (certain ones only) hence some of the problems. You may want to check it out.