i know a few ppl who have used the epi-no some liked it and some didnt.. i think it is a way to 'train' your pelvic floor and more importantly your perineum so you build up gradually to the birth itself...
I personally didnt use it - i did perineal massage, used evening prim oil too... and that was enough for me.. i did tear though (it was not as bad as i expected, i didnt even notice at the time) but i am not sure if the epi no would have helped me to be honest.. that is my experience. i have also heard of ppl tearing when using the epi no (v slight)... if it is recommended by your OB i would suggest having a chat to her about it and then do what you are comfortable with - we are all diff and whatever makes you feel most prepared is the best thing for you. in any case you dint need to decide anything about that kind of thing for a while! enjoy your pregnancy, all v exciitng!