What does Google Hangouts do well? Great way for communicating worldwide. It doesn't gives you problems and it works in most of the country. It's easy to communicate, it's stable. I have use other competitors, and it doesn't works well in some countries. Have try Hangouts, in Europe, USA and Latin America and it hasn't give any problem.
What does Google Hangouts not do well? I don't have complains of Google Hangouts. The version they provide for normal users works well, and the additional options for the people who uses google business is a plus. So you don't really think you're missing something like other services.
What do you wish you had known about Google Hangouts before using? I didn't knew the difference between the normal gmail accounts google hangout and the google business hangout, which is much better. This is something to take into consideration if you're planning to buy a email service, this a plus.
Have you used a competitor to Google Hangouts? If so, how does it compare? I have try Skype, but the service for latin america isn't really good. Issues in the calls, so that's why I prefer Google Hangout