Yes, you're firmly in the Google ecosystem with a Chromebook. Your music player is Google's player, your e-book reader is Google's. etc. Having said that, though, Amazon's music player and Kindle reader both work fine on it; note they are web-based apps. Most of the "apps" for the Chromebook are actually just URLs. This means apps don't consume storage space on the device other than the handful of bytes for the URL and the icon.
The main selling point? It's fast, lightweight, decent battery life and cheap. Does it replace the role of tablets? That's sort of a yes-and-no answer. Tablets are still smaller and lighter than chromebooks. Most chromebooks have an 11.6 inch screen (mine has a 14, so my battery life is a bit less than the smaller ones; that plus I have Bluetooth on all the time). They have real keyboards even though they're chicklet-style keys, which is good if you need to type much of anything and not so good if you don't.