hi, I have 2 pairs of fives. classics, which are my everyday ones and some spyridons, which are my trail running ones. I love them snd do all my gym work in them too.
you do have to learn a new style of running and transition very slowly to avoid injury. there is a lot of info on barefoot running on the fivefingers website and on the vivo barefoot website. also look on youtube for videos of the technique.
one warning though, it is bloody cold running in them in the winter. I only did a 2 miler gentle run and my toes were numb when I got home.