Thumbtack is a fairly big scam for contractors, for the simple reason that "clients" aren't required to make any financial commitment in order to post, so most requests are from people who never had any serious intention of hiring in the first place. They're "playing the field."
Yes, I have gained a couple of clients. Was it worth the money? Probably not, unless they really do remain very long-term clients. Here's what happens with Thumbtack:
1. You see a request and quote on it, which costs you.
2. Nothing happens. The client views your request, so you are out the cost. But they never act on it.
3. You are contacted by a scammer who claims all the other contractors are offering half the price (why don't you just take that offer, you think to yourself). When you contact Thumbtack, they claim you don't really know it's a scam.
4. If you're in the massage business, you get lots of explicit requests for sex. Thumbtack refuses to handle this one, and doesn't allow you to report them for what they are unless to you pay Thumbtack to send a quote, then report it as a scam.
How do I know the vast majority of clients never hire? Thumbtack sends a report on how you compare to other pros each week. If I actually got hired, which is very rare, I always, without any exception, have a better hire rate than every other pro. Meaning, getting hired once is the exception, not the rule. Oddly, after I posted this review I never got another one.
If Thumbtack starts charging clients to post a request, then it might be worth the money. Until then, I'd say don't waste your money. The only real reason for using the site is their SEO - I had one client who searched for my service, and the Thumbtack site actually came up as one of the top pages. So she placed a request and is now a consistent client. But this was kind of a one-off.