Have never tried bluetooth handsfree but I have been using Sennheiser ear buds for the past 3-4 months during my extremely sweaty 40-50 min treadmill workout. Though I have nt faced any prob so far but I am planning to get plastic ear cushions for them that are immune to sweat.
Before these I have been using original iphone 3gs handsfree and a4tech earbuds but their ear pads/cushions got ragged and started coming off so I would advise you not to buy these if you intend to use them during your workouts. Gray part around its ear pieces will start coming off within weeks.