PROS: Clear, nicely designed interface, Lots of customization options, Support for text, voice and video chats, Usual IM features: smileys, nudges, file sharing..., Extra tools like remote control, screen capture and video recording
CONS: Sometimes adding contacts is difficult, You can't download new interface themes, Number ID for contacts: not very user-friendly!
PROS: Great free software with all the bells and whistles of a paid program, Support for Twitch, YouTube, DailyMotion, Hitbox and other popular services, Huge library of plugins all available for free, along with the source code, on GitHub
CONS: Open source usually means slower releases for newer operating systems and hardware. Expect to have less up to date support