Usability testing online tools are kind of must have tools for every UX-oriented product / team. But there is one important thing to consider. The thing is in recruitment of participants. If you have a specific product for a certain type of users it is very hard to find test participants matching your persona. You will meet a lot of participants that will convince you that they are suitable for your test, but the fact is they are just people who are making money through such services.
My advise is: [LIST=1]
Clearly understand what result you expect.
Be as clear as possible in your questions and tasks.
[*]Scrupulously choose the participants.
My vote is for real user testings if it is possible. It will let you be more flexible, catch the pulse of the testing and feel the emotions.