I'm living in Germany where the male name
Jan is very common but I've just realized that pronouncing it the English way it sounds rather feminine. It makes me think of
Janet or
Janina or some ugly
Janelle or what not at once. But I guess it is okay as a boys' name, esp. if you consider that everyone has got a different taste concerning the pronounciation. I myself find it a bit too feminine-sounding and I don't like the whole sound of it anyway (sounds like 'jam' or something). But it's not a complete no-go on a boy, imo.
It would be even uglier if used for a girl as her full fn, in my opinion. As a nn for
Janet, it would be okay.