As a second entry into the Episodic style of gaming I'd have to say in terms of a full plate delivery Half-Life 2: Episode 1 does a one up on SiN by introducing larger environments and overall just a lot more variety into the mix as really the Half-Life universe seems to do in every release. The AI is refined and gives a pretty good challenge from start to finish, the environments are large and provide a good deal of fun to be had from them. For those new to the game they can also enjoy the benefits of HL2: DM and HL: DM Source as well.
For those new to the Half-Life universe I would advise taking a stroll through the original or at least Half-Life 2 before jumping into Episode One, for the average unknowing gamer the story might seem a bit off or full of holes but this is more attributed to the lack of experience that is given from playing through the original Half-Life 2. With that said Half-Life 2: Episode One has certainly given us a small glimpse into the epic battle that is on the verge of hitting gamers and for those looking to enter the HL2 universe this will be a must have item especially as cheap as it is going for online and in many retail stores right now.
We would like to thank the guys at Valve for giving us a chance to check out their latest work and look forward to Episode Two this Winter.