We have two Montague folding mountain bikes. We like them and use them a lot.
Of the boat bikes we have seen, they are the best.
I think that multi gears and fat tires are almost essential, and that full size wheels are highly desirable, if space on board permits.

We store each bike in an old sail bag, and spray the bike with oil from time to time. The bag avoids mess, and getting bikes tangled with ropes etc,

First, the good news
1) They are real bikes with real size wheels, 24 speed gears etc. It is just like riding a medium quality mountian bike
2) Frame is well made of aluminum, with medium quality Shimano gears etc.
3) Unfolding and installing the front wheel takes about 2 minutes each bike
4) Easily fixed with parts from any bike shop
5) They are rather bulky, so prevent us putting too much weight in the lazarette

The bad news
They are rather bulky, which is a problem on many boats.