Recs.
Updated
SpecsUpdate
Pros
Pro Free and cross-platform
Google offers all features of Hangouts completely free on iOS, Android and web with quick and reliable sync across all platforms. It's also tightly integrated in other Google services - it can be accessed from services such as Gmail, Google+, Chrome.
While the standard Hangouts offering is completely free, there are business, education and government solutions that increase the participant count to for video calls to 15 and offers support that may cost money.
Pro Group video support
If you have signed in your Google account, Hangouts allows making video calls with up to 10 people or one-on-one if you haven't. There are business, education and government solutions that increase the participant count to for video calls to 15 and offers support.
Additionally, you can switch between either the front-facing or rear-facing camera on your phone.
Pro Google Voice integration
Google Voice is a service that enables making and receiving free phone calls within US. Google Voice forwards incoming calls to all connected devices and allows placing calls from any of the connected devices, but they will show as coming from the Google Voice number.
Hangouts completely integrates Google Voice, allowing you to place calls and receive texts and voicemail from all of your device.
Pro Advanced status sharing
Hangouts can inform about the status of other users including what device they are currently using, whether they are in the middle of a call and when were they last seen. This allows making sure that you're not disturbing someone when initiating a conversation. It's possible to granularly turn off status sharing.
Cons
Con Lack of end to end encryption
Communications are encrypted between client and Google servers but not from sending client to receiving client. This means that there is still a possibility for someone other than the receiver to read your message if they have access to internal Google servers. But it allows also lightweight chat history search, spam protection and other features.
Con No way to moderate groups
If a member of a group chat room becomes disruptive, or there's some other need, there's no way in Hangouts to remove somebody from an existing chatroom. Similarly, if somebody is no longer active, there's no way to remove that user from the chat room to make slot for new active members.