I am a big fan of the original Half Life. I've beaten the game several times. It is one of my personal favorites. Now its sequels (Half Life 2, EP.1 and EP.2) are included among my favorite titles.
I really liked the episodes as well. They were too short though. The creators announced that in fact the three episodes (With the third one not released at this point) are just one long game. Of course that brings many negative aspects. For instance if you want to play all three of them, you may want to install them simultaneously, resulting in some big portion of your memory being occupied. But there will be plenty of time to talk about these games. Let's focus on this title for now.
This sequel had a lot of work. It had to surpass the original game. Half Life was a unique game. It has an amazing story, great characters, and was really entertaining. It also had a certain kind of "magic". It became a classic almost instantly. Its second installment kept a lot of the great details that made the original so good and improved them with other features that helped to create an immersing gaming experience. Half Life 2 is as good as the original.
The old characters returned, but with better skins. Some new and interesting characters are introduced. There new and cool enemies (Whose A.I. is good). The graphic detail is incredible. The water effect is by far the best I've ever seen in a game. They took a lot of time polishing this features and it shows. The only negative thing I can say is that this title is shorter than the original.
The plot is great also. It is almost too linear though. We now follow the adventure several years after the events of the first game. The population is being enslaved and is our duty to help them before annihilation. To make the game even more appealing, the creators added the spectacular gravity gun, which is an extraordinary weapon (Well, not a weapon per se). It has the power to lift heavy objects and neutralize their weight. This weapon is almost as important as the characters. We wouldn't be able to solve some of the most interesting physics puzzles without it, and is extremely fun to use. And I almost forgot, the physics are great.
It was extremely difficult to reach the first Half Life game in terms of quality, but I think that this game pulled it off. It has that certain "magic" also. And is highly enjoyable. I recommend this title. It might not disappoint anyone.