I know a lot of boys named Jan. I personally dislike the name and its sound. Too short and hard sounding for my taste. I prefer John or Johan or any other form.
Jan was the #10 name in Germany in 1980, #5 in 1985 (behind Christian, Michael, Sebastian and Stefan), #1 in 1990 and 1995, #2 in 2000 (behind Lukas), and #10 in 2005. It also ranked at #2 in the Netherlands in 1880, 1885, 1900, 1905, 1910, 1915, 1920, 1925, 1930, 1935, 1940, 1945, 1950, 1955, 1960, 1965, and 1970 (behind Johannes every time). It dropped down to #3 in 1975 (behind Johannes and Jeroen), #4 in 1980 (behind Johannes, Jeroen and Dennis) and was back at #3 in 1985 (behind Johannes and Dennis). It fell to #8 in 1990, and was out of the top 10 by 1995. In Norway, it ranked at #6 in 1935, #1 in 1940, 1945, 1950, 1955, 1960, 1965, and 1970. In 1975 it was at #3 (behind Thomas and Lars) and #5 in 1980 (behind Kristian, Thomas, Lars and Stian) and it was out by 1985.
I really don't feel this is more a girl's name than a boy's name. I think only UK, US and Canada people might see it as a girl name, whereas in most other countries where you find the name, it's a variant of John and Ian, so it's clearly a boy name to many!
I know this has been a form of John for a long time, but I think it's too feminine-sounding to use on a boy. It also seems like Jan is too masculine-sounding to use on a girl, so I can't see this becoming popular for either sex.