Free to use for clients, and a pay-as-you-bid for jobs system for businesses, the site leans a little heavy on the vetting the businesses while leaving those asking for bids a free reign to be flaky. Only a problem if you find you're paying to make multiple bids on dubious jobs. (They do offer a 48 hour "no response" refund of the biding fee.) The costs are not prohibitive but businesses would do well to hone their crazy client senses.
I look forward to exploiting Thumbtack.com for all it's worth.