-Quick Review-
Half-Life 2: Episode One was Developed by Valve Corporation and released 2006. Not much changed from Half-Life 2, this game is a continuation and starts right after the events of Half-Life 2.
http://steamcommunity.com/sharedfiles/filedetails/?id=520555541


-Detailed breakdown review-
Story: After the events that took place in Half-Life 2, Gordon and Alyx Vance are reunited and need to escape City 17. Unfortunately, they cannot escape before the Citadel's core detonates. So Alex and Gordon are forced to enter the Citadel to stabilize the core long enough for them to safely escape.
http://steamcommunity.com/sharedfiles/filedetails/?id=520555575

Gameplay: There are only two changes to the core game: First, Alex's AI has been dramatically improved, so she is more helpful and not nearly as annoying as most NPC's that simply repeat the same phrases and actions. Second, you start the game with the Gravity Gun, which is utilized both in combat and puzzle solutions.
http://steamcommunity.com/sharedfiles/filedetails/?id=520555594

Achievements: There are 13 achievements in Half-Life 2: Episode One. Five achievements are unlocked by just completing the game, the rest are achieved by special circumstances. (like... “beating the game firing exactly one bullet”, “not killing any stalkers”, or “killing an elite soldier with his own energy ball”)
To be honest, these achievements aren't very bad, this was a relatively easy 100% completed game, compared to others games.

Price: Half-Life 2: Episode One, is priced at $7.99, unfortunately the game is very short, and this price is a bit high for the amount of time it will take to play through the game. (You will probably spend about 5 or 6 hours to beat it)
A far better alternative, is to purchase The Orange Box for $19.99, you will purchase: Half-Life 2, Half-Life 2: Episode one, Half Life 2: Episode Two, and Portal.

Conclusion: Story: Still very good, but unfortunately much shorter then Half-Life 2. Gameplay: Pretty much the same as Half-Life 2, the only difference is you use the Gravity gun much more, and much sooner. Achievements: Straightforward, fairly obtainable 100% completed game. Price: Over priced, wait for a sale, or purchase the Orange Box for the best value.
I recommend this game to anyone who played Half-Life 2 and wants to continue the story of the Freeman...

Related reviews
Here's the URL to see my review for Half-Life 2
Here's the URL to see my review for Half-Life 2: Episode Two
Here's the URL to see my review for Half-Life 3 ...(...is what I would say if it was released yet *sob*)

[u][b]-Quick Review-[/b][/u] Half-Life 2: Episode One was Developed by Valve Corporation and released 2006. Not much changed from Half-Life 2, this game is a continuation and starts right after the events of Half-Life 2. http://steamcommunity.com/sharedfiles/filedetails/?id=520555541 [u][b]-Detailed breakdown review-[/b][/u] [b]Story:[/b] After the events that took place in Half-Life 2, Gordon and Alyx Vance are reunited and need to escape City 17. [spoiler]Unfortunately, they cannot escape before the Citadel's core detonates. So Alex and Gordon are forced to enter the Citadel to stabilize the core long enough for them to safely escape.[/spoiler] http://steamcommunity.com/sharedfiles/filedetails/?id=520555575 [b]Gameplay:[/b] There are only two changes to the core game: First, Alex's AI has been dramatically improved, so she is more helpful and not nearly as annoying as most NPC's that simply repeat the same phrases and actions. Second, you start the game with the Gravity Gun, which is utilized both in combat and puzzle solutions. http://steamcommunity.com/sharedfiles/filedetails/?id=520555594 [b]Achievements:[/b] There are 13 achievements in Half-Life 2: Episode One. Five achievements are unlocked by just completing the game, the rest are achieved by special circumstances. [i](like... “beating the game firing exactly one bullet”, “not killing any stalkers”, or “killing an elite soldier with his own energy ball”)[/i] To be honest, these achievements aren't very bad, this was a relatively easy 100% completed game, compared to others games. [b]Price:[/b] Half-Life 2: Episode One, is priced at $7.99, unfortunately the game is very short, and this price is a bit high for the amount of time it will take to play through the game. [i](You will probably spend about 5 or 6 hours to beat it)[/i] A far better alternative, is to purchase [url=http://store.steampowered.com/sub/469/]The Orange Box[/url] for $19.99, you will purchase: Half-Life 2, Half-Life 2: Episode one, Half Life 2: Episode Two, and Portal. [b]Conclusion:[/b] Story: Still very good, but unfortunately much shorter then Half-Life 2. Gameplay: Pretty much the same as Half-Life 2, the only difference is you use the Gravity gun much more, and much sooner. Achievements: Straightforward, fairly obtainable 100% completed game. Price: Over priced, wait for a sale, or purchase the Orange Box for the best value. I recommend this game to anyone who played Half-Life 2 and wants to continue the story of the Freeman... [b]Related reviews[/b] Here's the URL to see my review for [url=http://steamcommunity.com/id/atratzu/recommended/220] Half-Life 2[/url] Here's the URL to see my review for [url=steamcommunity.com/id/atratzu/recommended/420/] Half-Life 2: Episode Two[/url] Here's the URL to see my review for Half-Life 3 ...[i](...is what I would say if it was released yet *sob*)[/i]