Very short, but equally sweet.
The first thing you're sure to notice when you start the game for the first time (if you have a newer computer, that is) is the addition of high dynamic range lighting, commonly referred to as HDR. This graphical effect adds an enormous amount of style to the already spectacular-looking Source engine. The eerie lighting in both the Citadel and the darkened residential buildings of City 17 looks better and more realistic than ever due to the complete integration of HDR in Episode One.
In the previous two Half-Life games, not only did our hero refrain from verbalization (he doesn't even groan when he gets shot) but also companionship. This changes in Episode One, now that Alyx accompanies you throughout the game. There will be points at which Gordon must fight and work on his own, but these sequences make up only a minority of the game. Disappointingly, however, Alyx has only minimal influence in any of the "boss fights," mainly keeping a few antlions off you now and then. When it comes down to it, though, Alyx leaves the heavy lifting to the silent partner.
Tactically, it's a little too easy to abuse the leniency the game gives to Alyx with regard to her health points and ammunition count. On the normal difficulty setting (not the default setting of "easy") I never wittnessed her die or run out of ammunition. Often times, this leads to sitting back and watching Alyx pump round after round from her shotgun into the oncoming zombie hordes. Out of respect, you'll always end up helping her with the combine zombie that's mauling her, even though you could just as well wait for him to pull his grenade and obliterate half of the onscreen enemies, leaving Miss Vance seemingly unharmed and only momentarily phased.
Regardless of the flaws it has, Episode One delivers what it really needs to, and with flying colors. Honestly, the only reason you should spend your money on this game is for the story. Despite the amazing visuals throughout the short campaign, the focus and purpose of the episode is the development of the Half-Life plot. For $20, one might expect a little more story than Valve delivers in Episode One, especially since the game engine has had only a minor aesthetic retouch since it's predecessor.
Overall, Half-Life 2:Episode One is an enjoyable and familiar romp through Half-Life lore, and if you're interested in a good science fiction story or a great-looking, albeit short single player puzzle-solving shooter, Episode One is definitely worth a look.