For my projects I've using the library LatLongLib, from Ahmed Taha. I think that it's very easy to convert coordinates from the UTM system to the Latitude-Longitude system and vice-versa. You just need to play with the classes UTMUtils, UTMPoint and LatLonPoint.
Time ago I also considered choosing Jcoord. It was easy and straight to the point too. However, I needed to use the WGS84 ellipsoid and, at that time, only LatLongLib seemed to have that feature.
