On the site that I used, it said if you made under $50,000/year (with a few other stipulations, which I'm not sure what they are) then it was free. But the one I did only let me do my federal taxes free. I paid $9.95 to get my state done free, but I figured with the convenience, and the fact I was getting back over $500, it was worth it. Especially since it was all online and really easy to fill out and submit. No hard copies to send in the mail and no forms to get. It filled it out for you and made it into a PDF when you were done (so you could download them and print them out for your records). Also, you can have your money deposited directly into your bank account so that you don't have to worry about the check being lost in the mail and you get it faster.
Once I'm making normal amounts of money (i.e. over 50,000) I think I'm going to get an accountant to do my taxes. I hear it's only like $100 and I figure if my taxes are really complicated by then, with mortgage interest, investments, etc., it will be easier and worth the money.