I really enjoy having them, but they need a bit of work.
Let me paint you a picture:

You've lined up 8 videos and start watching them. You go to fullscreen and watch a couple of videos.
Youtube gets points here for the smooth transition between videos while in fullscreen.
Now, you need to exit fullscreen. In the middle of the video. With the video still running because it's a standup, say.

Youtube does a full refresh after exiting fullscreen. Why?!

It totally kills the mood. No matter how fast you connection is, you still get a second of pause. 

The funny thing is that the flash player is already playing the correct video. All they need is to repaint the DOM. They could do that on the fly, with Javascript. And they could use pushState to change the URL, as well.

Youtube are already doing a great job with Cosmic Panda, I wonder when they'll give this some attention.