I have been using Google Chromecast for over 4-5 months and I can say that its the best piece of streaming device available in the market.
Pros:
1. Simple setup, just plug and play.
2. Stream from multiple device and with the new update, it can stream from devices which are not connected to your wifi.
3. Regular updates from Google
4. Tons of Chromecast app available in the Google Play market. You can stream video/audio,play some simple games, draw on the TV and many more. https://www.google.com/chrome/devices/chromecast/apps.html
5. Can stream from local media stored on Phone/PC/NAS/etc
6. Deeply integrated with Google and Android.
Cons
1. Requires internet connection to start on. You can continue to stream from local media even when there is no internet connection.
2. Few formats are not yet supported but then you can plenty of apps which can circumvent this restrictions like plex.
Hope it helps