1. Do you use such services? If so, what do you choose?

I used UserTesting in the past.

2. How you choose it?

I used it to test new feature ideas and to refine existing UX.

3. What was the main problem of using such services?

It got kind of expensive for us (we were a startup) and sometimes the users were not as targeted as we wanted (especially when we were testing b2b stuff).

4. If you didn't use it, tell me please why?

5. Will you use such services in future?

I now work at a large company so probably not since we have researchers on the team who do their own recruiting and testing. That said, I find the insights from DIY things like userTesting to be good enough especially if you've done testing before and have some experience with user studies and UX design or product management.