It is a very non user friendly application. Very poorly designed, graphically speaking, the icons and names of the buttons are not intuitive, you couldn't deduce thinks by just looking at them. I never found the option to start a call properly, always did it using commands (Ctrl + m).
I didn't like that whenever I started a new call I couldn't ended it until I set a "Voice disposition", which is the option to select why the call was ended. Which means that if I or the other part ended a call, in Five9, the call remained technically "open" until I selected the voice disposition.
The calls entered automatically so, suddenly you were in a call without knowing (Yay!).
My partners that called to my number often complained that the call was routed to someone else in the office (our extensions not always routed the call to the proper person that it was supposed to be routed).
The application didn't allowed us to call numbers in some countries like Israel or India.
My boss also told me that is very difficult to calculate the logged hours.