So I had my taxes done today and let's just say I kicked some butt

Anyhow, my new company offers a Flexible Spending Account and I am thinking of signing up. I am only looking to put about $100 a month in (I get paid monthly) since I got killed last time I had to go to the dentist, but I have some questions. I know it is use it or lose it, but are you taxed on the funds you don't use? Also, do you have one? If you do, did you find it was worth doing?