Really good character elements and some interesting new HL2 scenarios; too bad it was so short.
It's pretty much just more Half Life 2 and it's considerably shorter but the character development goes deeper and in many ways makes the game more interesting. By the time you finish Episode 1 you will probably be wanting an Episode 2 pretty badly. I've been waiting for over a year and now that the Orange Box is out I can finally see what happens next. If you don't have any Half Life 2 games then pick this (Episode 1) up as part of the Orange Box; you will want to play the original HL2 before it and Episode 2 when you're done for sure.
Oh, and if you've never seen or played Half Life 2 before then you should know that this game is technically the same in almost every way:
Excellent graphics, very modest hardware demands, widescreen supporting, single player first person shooter. Episode 1 doesn't require Half Life 2 to play, it's not an expansion.