Netflix will automatically install as a tile on your Windows Media Center Start screen, in the Movies section. If you have a new OS installation, and still don't see it, you can force a download by going to Tasks>Settings>General>Automatic Update Options, and manually updating.
On September 15, 2015, Microsoft dropped support for the Netflix Media Center add-in and removed it from users' systems who updated on or after that date. 
In 4th Quarter, 2015, a WMC user (Marc Davis) began writing a replacement add-in. Netflix for Windows Media Center
Pros and Cons of Watch Instantly
Unlimited - Watch all the movies that you want for $8/month Integrates into windows media center Video is good quality (depending on connection speed) Audio is good quality Conveinence - Instant access to movie library and thousands of movies No late fees Free One Month Trial
Very dependant on your pc's hardware Very dependant on your internet connection speed. Poor selection of NEW releases No closed-captioning
Netflix, Inc.