I used Thumbtack once, and felt that Thumbtack was a scam. It costs me $2-4 just to send a quote. I spent about $20 sending quotes over the course of a month and never saw any business. A lot of the requests I received just seemed fake (e.g. $3000 commercial shoot requests with no details). I'd actually prefer to pay more and give Thumbtack a percentage of the deal rather than pay a lower cost upfront. At least that way they have motivation to verify customer requests.
I recently used Thumbtack and had a good experience. Unfortunately, there were no movers in my area so it was a short-lived deal. After a day or so they e-mailed asked me to take a survey and I obliged. I noted in the survey that I wasn't able to find anything in my area, but I liked the site UI. A few days later I got a human-written e-mail with a list of moving companies in my area.