Hi,
1) Whether it's "worth it" is up to you. I can only answer specific questions. Perhaps THIS will help:
http://www.tomshardware.com/news/nvidia-shield-remote-gamestream-review,26476.html
2) Gamestream is using the Shield device to stream games from your PC to it, or through it to your HDTV whereas GRID is NVidia's Cloud Network solution.
So for Gamestream your computer is the processing device you receive video from and upload key presses to, and GRID is a server somewhere.
http://shield.nvidia.com/play-pc-games/
I can only assume that you'd have access to GRID by purchasing a subscription to a service that uses it. It's not for accessing games you already purchased.
http://www.engadget.com/2013/12/04/nvidia-grid-open-beta/
Cloud computing tends to have issues, especially when you become more distant from the PC/Server but you'd have to experiment with it yourself. Also, some games are easier to play with a higher network delay than others depending on how much time you have to respond to a situation.
3) The Shield device also has Android OS that supports local games that fit on the flash memory just like Tablets.
4) *Find out your monthly BANDWIDTH allowance in GB, then divide that by what the Shield would use to see how many hours it would take to max out your allowance. You might be surprised at how few hours you can game.