Worth playing if you're interested in finishing all of the Half Life installments.
-same fun, fast paced gameplay from Half Life 2
-new team based "co-op" feel with Alyx is a nice change
-overall AI of Alyx is impressive
-same immersive and detailed environment, incredible visual design, and great voice acting
-a couple new enemies, such as the "Zombine"
-fixes some minor annoyances from HL2 (e.g. O2 and flashlight power no longer become depleted super fast)
-adds the ability to sprint
-the constant city surroundings can get dull
-Alyx's dramatic reactions are sometimes corny or annoying
-gameplay continues to focus on shooting more than puzzles
-not really any new weapons or gadgets
-no major progression regarding the plot
Episode One doesn't really offer anything new in terms of both gameplay and plot, but it's worth playing if you're interested in finishing all of the Half Life installments. It's also quite short, but then again, it's called an "episode" for a reason.