This app is a must have for all windows phone users, since it allows you to play those YouTube videos that are otherwise unavailable to mobile users because you have to watch an ad before it (hey, Google, if you are reading this, please come up with an official solution for that.) This works not only with the YouTube website, but also with embedded YouTube videos across the internet.
The main advantage of this app, however, is that you can watch videos from Megavideo, videobb, etc. Something that so far, only Android users could do (or iOS users if the corresponding app exists.)
I tested the app myself with videobull.com and found that it does work, I was able to watch mythbusters on my phone! (Sorry for the low quality video, I did not have a great camcorder with me)
Metro Browser works by acting as an intermediate layer between the site and the IE control it hosts, and figuring out when you have a flash video to be played. As soon as it detects one, it plays it using the player. They do not seem to be using the wp7 video player, and you have to wait for a while for the video to be buffered before you can play the video.