I do them myself rather than pay someone, because I'm usually broke.
I think most tax preparers will defer payment until after you recieve your refund. Not that I'm suggesting you shouldn't do them yourself, if you know how, which I guess you do.
I've always used someone. I think my father did a few for me when I was still in school, if I ever earned enough money over summer jobs to have to file, which I can't remember at this point. But after school I started sending them to my mother's accountant (he's actually her former accountant, her taxes got too complicated when she became a landlord). I pay him less than $200 and always get several thousand back. I think I used to not pay him until I got my refund, before I'd been working long enough to earn a salary that allowed me to afford paying at the time of service.
I have to send them off soon, actually, because now they are going to be more complicated - for years I only had a job and a savings account, but eleven days before the end of 2007 I became a homeowner, and a complicated one at that, although I'm sure it's nothing he can't handle - I own a co-op, meaning I have deductability through my maintenance payments (which cover, among other things, property taxes) as well as my mortgage. Also, I co-own the place with someone I don't file jointly with, and I have no idea how difficult that is to deal with. Mr. Wildwoman (he is not legally Mr. Wildwoman, hence the seperate filing) does his own, always at the last minute (literally, going out to the mailbox on the night of the due date). If we ever do get married, I'm insisting on using the accountant.