The user interface is very easy to navigate and I love that they have many different features like voice call in or out calls, group chat with up to 50 people at one time, video streaming with my desktop or mobile device via WiFi or 3G/4G data connection, etc.
It's also pretty cheap for what you get compared to other services which can cost $30+ per month (or more). Nothing really comes to mind as a favorite feature but this service has been working perfectly well for me so far! If you're looking for an inexpensive way to communicate with multiple people at once then iPlum might be worth it. We are using iPlum primarily for our team meetings since we don't always want everyone else around us to hear every little detail of our conversations.