I've never had any trouble with Watch it Now. Silverlight was a really easy installation. I never have any problems with buffering for the movies, but I do have a fairly fast internet connection, so I can't be certain how it fares on slower connections.
As for throttling, I've never heard of any issues with it, nor had any issues myself. I watch a lot of rare independent movies, and my dad watches all of the latest blockbusters, and movies on our queues are almost never out of stock, and when they are, they've always been in stock with-in a few days. My dad has been getting major releases, such as Inglorious Basterds and Star Trek, in our mailbox on their release day.