Wikipedia has an article on power naps in which they mention the "caffeine nap". This is short nap, where you take a cup of coffee before the nap, and then benefit from the combined effect of the nap caffeine hit after the nap.
The generally recommended length of time for a power nap is about 15-30 minutes, any longer and you're likely to end up in deep sleep and feel terrible when you get up.
I've never been a big power napper, but I noticed it certainly can be refreshing, if you have the time or opportunity to take naps during the day. On long car journey's I'd definitely recommend ADDers take a nap if they're feeling tired, as sustaining attention for long periods of time isn't something most ADDers are particularly good at.