Personally I use Internet Download Manager which by default gives an option to save audio and video when you view it. It's how I save all YouTube videos because of VM's terrible streaming problem
Once the vid is fully buffered then I just copy it out of the browser cache, although my streaming speed is more than adequate even for 1080p
One problem I do have however is some new 480p and most new 360p vids are split into multiple files of less than 2MB in size in the browser caches and I haven't found a way of recombining them into a single file. Guess a download manager might be able to do that though...