These three videos show Union from start to finish for those who are stuck in a particular part or just curious. However, I do strongly advise against watching them if you intend to play Union at some point as they will inevitably spoil some of the surprises I have carefully built into the experience.
If you haven't played Union yet, what are you waiting for?! The maps are available from ModDB now and all you need is Half-Life 2: Episode Two!
And don't forget: I'd love to hear your thoughts afterwards even if you hated it!