Rainforest was in use at my current company prior to my starting in lieu of having an in house tester.
I think it has some interesting possibilities and common drawbacks.
Fundamentally it’s crowdsourced scripted test case runs. You provide the test scripts and the browser targets and they provide the testers. Because it’s distributed you can get a lot of coverage fairly quickly across multiple browsers. They also did multiple testers per test to try and limit false positives.
But… it’s scripted test cases run by people who have no knowledge of your product (likely) or industry (probable) with unverifiable credentials or skills.
We were on a very low end tier and could have used up our credits incredibly quickly. On top of that it meant having a full time person to manage the existing test suite, create new tests, manage environments, and manage test runs. So, I could have the expense of that full time employee, plus the expense of our Rainforest package (more than it costs for a good tester where I am), for something we could run the equivalent of a couple days a month.
They also have an exploratory testing package they tried to sell me on. That’s much more interesting to me, to have fresh eyes on a feature or functionality, but I couldn’t justify the price. Have Rainforest do a day or two of exploratory testing a month for me or hire a full time person and have tooling / training budget left over… Easy call for me.
They worked hard to retain me, but when I told them I didn’t have the capacity or desire to continue using them they did terminate our contract early. That alone means I’d consider them in the future, given the right circumstance.