As you probably know that there is no official YouTube app for Windows Phone. But there are some third party apps in the Windows store for YouTube. ‘Metrotube’ is one of them. I use this app to view YouTube videos on my Windows Phone. You can find this app on Windows store.
The app is very light weight and has some good features. I can view videos both in small and in full screen mode. The app has two themes, light and dark. The dark theme is good if you are watching videos at night. After logging in to the app with my Google account, I can access all my subscriptions and personal uploads. All the main menu options which I can access from YouTube website, also I can access from this app (like ‘popular in YouTube, music, movies etc.’). I can also off the video and play in only audio mode with this app. I can share YouTube link from this app with email, twitter and many other options.
Over all if you have a Windows Phone and watch YouTube often then I recommend you to try this app once. It is a good app in Windows store for YouTube.