Half-Life 2: Episode 1 (Which we shall call HL2E1 from now on) has the same gameplay and graphics as Half-Life 2. So why buy it? Here's why.
Being the follow up to the game of the decade HL2E1 starts where Half-Life 2 ended (Which i shall not say due to spoilers). The plot involves you being in the ruined and dilapidated remains of City 17. Your task is to infiltrate the citadel and shut down it's power core, otherwise City 17 will turn into one very large crater.
As you enter the citadel, Alyx Vance is by your side. Now, in my opinion Alyx has to be one of the best characters in gaming. As an Ai, Alyx doesn't get in the way of your shots, she stays in cover, and is generally helpful. As a person Alyx grows on the player, as she is never rude, or aggressive towards the player, and as you continue to play, her emotions towards Gordon Freeman begin to evolve, where looks of worry appear on her face as she tells Gordon to be careful.
In conclusion, HL2E1 is a great follow-up to Half-Life 2. The characters continue to evolve, along with the story, definitely worth a buy.