It's well known that Half Life 3 never came out, but I was surprised to know that "Half Life 2 Episode 1" was an episodic sequel so it is basically the 3rd game. However, this episodic sequel was meant to have 3 parts, but the final part never came out. So both Half Life 3 and Episode 3 never materialised, which surely adds insult to injury.
Alyx didn't feature that much in Half Life 2, but played a major role in Episode 1. She is a companion throughout this game too but there's sections without her. Early on she is injured, so you get a Vortigaunt companion and travel through Antlion-infested caves, and come across a new variant which launches acid. Once Alyx is healed, you go to acquire a vehicle and travel with her to a few locations.
I thought there were some varied and interesting ideas in Episode 1. There's decent levels here but they don't seem as tense or original. It generally seemed a more straight-forward experience. The final battle is interesting though, with you taking down several Striders which are accompanied by Hunters, as they march towards your allies' base. Rather than taking them down with rockets like you did in previous instalments, you now launch a new invention; "Magnusson Balls" which takes them down in one go. Getting close and getting an accurate hit is the challenge, and you need to navigate the area in your car and transport Magnusson Balls.
The episodic format does mean that Episode Two is a shorter Half-Life experience, but is a bit longer than Episode One, at around 5 hours.