We've had an excellent experience with Testlio over the last 3+ years.
Some of the "table stakes":
- thorough testing
- clear bug reports with screenshots, videos, and steps-to-reproduce.
- integration with bug-trackers (Jira in our case)
Testlio delivers on all of the above, but I've been most impressed with how flexible their process is and how excellent the project managers are.
In terms of test cases and management of the testing cycle, you can be as hands-on/off as you like. Testlio's PMs will do a great job of putting together test cycles and test cases with minimal input from us, but they will also accept and integrate as much detail as we can provide. Sometimes we provide a huge list of test cases for a new feature and the testers execute them all. Other times we say "try anything that you can think of that touches feature X". We get quality results either way.
Over the years we have made a number of times to how and how often we request testing. Typically a quick email or call with our PM is all that's required and then Testlio adapts their process to fit our new requirements.
Finally, Testlio has no trouble accommodating one-off requests for extra testing on a specific type of phone or operating system, language, etc. We get a good mix of all of these without asking, but we can always request special emphasis on any of them if we need to. Review collected by and hosted on G2.com.