From /r/todayilearned : http://www.reddit.com/r/todayilearned/comments/26xd2h/til_menthol_in_cigarettes_serves_a_purpose_other/

Menthol apparently inhibits metabolism of nicotine. This essentially means that your body gets rid of nicotine slower -> nicotine stays in your body longer. Anyone experience this with e-juice? Personally I've always found menthol in juice to be really nice and I enjoyed vaping it more (with nicotine).