What do you like best about the product?
It allows to setup a test, get result, analyse them and act on them extremely fast. If you were to do real world lab test the process would take weeks, here it usually takes me a day from test ideation to analysis.
I like that when a test doesn't go the way it should have been (poor audio, testers tested the wrong thing, etc) UserTesting.com will always replace the test with another one in a matter of minutes. And because it's so fast to get something tested, the lost time does not make any difference into my day. It's an excuse to get another coffee.
I also like that you can do split groups (A/B) to test variations of design and that User Testing will randomized in which order they are seen to remove bias.
What do you dislike about the product?
The users in UserTesting default panels sometime seem like experts. Because they know the tool and are paid to use it they are de facto professional testers. When something seems easy to them I like to take it with a grain of salt - and build another test.
Conducting remote testing allows to gain efficiency but it requires to write test scenario a certain way, to make sure that you don't create bias or send testers the wrong way. It's a skill that needs practice.
Recommendations to others considering the product:
Always to a dry run of your test (launch one test) to make sure that the language is understood, tasks are in the right order, prototype does not bug, then roll out to another 5 testers to get the right number of testers per Nielsen Norman recommendation.
What problems is the product solving and how is that benefiting you?
I have been using UserTesting to validate concept, validate problems, validates flows. It allows me to: select the right experience, enhances experiences, prioritize the right problem to go after.