TLDR: A must-play for anyone who loves Half-Likes and that particular indescribable but magic vibe all good Source Engine games seem to share. However, my recommendation is to buy on sale because this game is short - took me under 3 hours to complete it.
----
SiN Episodes is a huge improvement on the first SiN - it retains the same world and goofy Austin Powers style of humour, but the level design is vastly improved. You can see your final destination right from the start of the game and vice versa, but the route to get there is weaving and intricate. This kind of cohesive level design really adds to immersion as you get a sense of believable scale when you can see exactly how far you've climbed through the landscape.

Gameplay is much like Half Life 2 but with a heavier emphasis on combat and only some very basic environmental puzzles.

The really sad thing about SiN Episodes is (also much like HL2 in fact) - they never finished the story. The episodic format of game releases was a huge fad in the late 00s, but clearly failed as a business model for many developers. For this reason, you have to go into this game knowing in advance it never got finished. I would guess the one Episode we got is only a third, perhaps even a quarter or a fifth, of the "full" game the devs planned. It's such a shame as this was a really fun title and one of the best Source engine games you can play aside from HL2 itself.

Everyone knows it's a game built off of the source engine, so it feels like Half-Life 2, of course. This game also has Red Faction (2001), Far Cry (2004), Deus Ex (2000), and F.E.A.R. (2005) vibes. It's just got that early to mid 2000s vibe to it and love it for that. The game has actually a lot of cool stuff to it, i kinda wish the franchise wasn't dead because the world also feels alive which is amazing for a game made in 2006, and it does it better than most of the modern AAA titles out today. The game also has a great soundtrack and also the reverb yelling from the enemies sounds very realistic for the time this came out, i dont think i have heard anything as good as this sound design since like the battlefield games. The game is short, feels like a giant tech demo but it's actually pretty fun and it's only worth buying for like a buck or two. Thankfully once you can beat the game, you are given the option to freely play a arena mode to make your moneys worth i guess.