After completing Half Life 2 I made the transition over to playing Episode 1 mostly because I liked how Half Life 2 picked up the pace towards the end of the game.  Well Episode 1 does move faster than the beginning of Half Life 2 and having Alyx (AI character) going through the entire episode with you makes the experience more enjoyable.  For the most part Episode 1 is a return to the puzzle dominated game play with very few run and gun moments.  There are some nice achievements that help keep the game a bit more challenging in some areas and a bit more fun in others. 

Episode 1 is the first true attempt at episodic gaming where instead of getting a meaty 10 or 12 hours of game play you get a shorter stint with more frequent iterations.  Most AAA game developers take their time to create a new game.  Often you'll see multiple years between releases.  For instance Halo came out in 2001, Halo 2 in 2004 and Halo 3 in 2007.  To wait three years for a sequel is a long time but often occurs in the gaming industry. 

So Valve, developer of Half Life, have started releasing "episodes" of Half Life to help keep their fans feeling like they don't have to wait a decade for another game.  Episode 1 took a little over three hours to complete and it appears that Episode 2 will be slightly longer.  The model is not as fast as some would like but it keeps the content coming at a faster pace than before.