(Note: for continuity purposes, Half-Life 2, as in, the one I also have for the PC, will be referred to as Half-Life 2 Episode Zero.)
Episode One is not as good as Episode Zero.
It basically recycles the Citadel and City 17 portions of Episode Zero. No new weapons are introduced, but one new enemy appears: Zombines. They're like the regular headcrab zombies, except they're faster, tougher, and can do suicide runs with grenades. Their kamikaze attacks make them dangerous.
Also, as most gamers love to point out now, Episode One is much shorter. Now granted, a short length is not necessarily a bad thing. However, to follow up what I wrote about Portal, I did realize this: even if you replay a short game, you'll finish it faster than a long game. You can memorize a short game more easily than a long game. Episode One, effectively, can be played to death more easily than Episode Zero. Whether or not that's a good or bad thing is left up to the player.
And also, Alyx tags along for most of the game. For an AI controlled controller, she's really smart. She'll run away from grenades, take cover, aim for their heads, and can even do a kick for a melee attack. She's good . . . too good. She has unlimited ammo, and although she can die, (thus causing a game over,) unlike you, she can regenerate lost health when not in danger. Yeah, she has the Halo-like lifebar, and you don't. If you keep her out of danger, she's practically a gamebreaker in sections where she appears. The parts when you are alone are obviously the most challenging segments, for you have to rely on yourself here, much like in the earlier games when you couldn't recruit a security guard or soldier to help you out.
Still, it's Half-Life 2. It continues the story, has excellent level design, the Gravity Gun (from the start this time,) top-notch voice-acting, and unbelievable graphics. And even if Episode One disappoints you, remember, it's practice for Episode Two.
