Has anyone ever hired a publicist to promote their film?

I haven't really looked into it because I assumed I would not be able to keep up the publicist's fees.

Perhaps I could. Are they worth their fees in general?

Blogs are being used for such publicity, for current example you search for "Gossip Girls" you will find many positive review about this show written by bloggers, that making it so popular, contact program like sponsoredreviews.com etc they arrange for paid review which are quite affordable and cost effective too.

Nope. Are you trying to publicize to distributors or to audiences? If the former, then what you'd rather have is an agent or rep to shop your movie to distributors. And you wouldn't pay them any fees. They'd get a percentage of your deal.

If the latter, then what you want is a marketing or ad firm.

I think it attract both, I forgot to mention social networking too, since blogs and SN sites are interactive in nature and are greatly influenced with user generated content, these platform seems most suitable for promotion especially those having tight budget, however you need to target your specific audience through little research.