Episode One is the equivalent of 4 best hours of Half-Life 2, save the vehicle rides. The review is spoiler-free.
In case you haven't played Half-Life 2, it's highly recommended that you do, if you plan to play Episode One. The episode's beginning is the epilogue of Half-Life 2, and you'll be left walking blind in terms of story, unless you want to play this expansion pack for gameplay's sake.
As soon as the game starts, you'll notice that the Source engine, the same engine that powers a game from 2004, is far from outdated. Valve has added some dramatic effects to characters and the environment, but they haven't overdone it, like some of the games currently on the market. Apart from the aforementioned minor changes that add up to the gorgeous environment, Valve has used HDR lighting to not only improve visual quality, but to mimic our eyes' reaction to light. If you go through a well lit area into a dark one, everything is going to be pitch black for a moment, until your 'eyes' get used to the darkness, and vice-versa.
Another aspect of the game that stands out the moment the first level starts, is sound. This follow-up of Half-Life 2 features top-notch voice acting, that brings characters to life and makes them sound incredibly believable. On top that, the dialogue is brilliantly written. In fact, such dialogues can be rarely seen in a first person shooter. Such Hollywood-quality voice acting and dialogue script are mostly seen in adventure games.
Another great thing about sound in this expansion pack, which deserves its own paragraph, is music. Ominous and dark areas will have low-beat music, while some fast-paced beats will boost your adrenaline as you battle your way through City 17.
The gameplay is almost identical to Half-Life 2 - you even pick up the same weapons that we have seen in Half-Life 2, bar the bugbait (as it would make this ride way too easy, after all, a good part of your enemies are the antlions). however there is a new type of enemy - the long-awaited zombine - a headcrabbed combine soldier. This new type of zombie moves faster than the average zombie, but significantly slower than the fast zombie. It can also take out a grenade and charge at you, holding the 'nade. You can then either snatch the grenade out of its arms with the gravity gun, or make the zombie drop it by dealing some damage to it.
What really makes Episode One stands out, though, and what makes it fun, are scripted sequences, hilarious remarks and most importantly, puzzle intermissions, which are placed in excellent spots. As soon as you feel like the combat is dragging on, you're rewarded with some soothing puzzle-solving. Nothing too hard, but nothing too painfully easy.
Scripted sequences are brilliantly done, but just like puzzles, they're scattered throughout the game. The episode isn't just a journey through scripted scenes. Unfortunately, I can't describe them or give you an example as that'll ruin the experience.
Finally, the length of the game. It'll take you from 3 to seven hours to finish the game, and that depends on your skill and whether you rush or take your time to listen to characters and whatnot. Whilst very few questions have been answered (a couple only, actually), the ride is really worth it. Valve has once again shown what great game developers they are. They have achieved making the players want to play Episode Two the moment Episode One ends. Hell, they have made me - the player - want replay Episode One, and due to the commentary system - it's worth it to replay the game. Get it, you won't regret it.