Recently played SiN for the second time and had a total blast with the game. Thanks for that, Ross! I'm very excited about the upcoming remasters of SiN and Kingpin, which unfortunately are now delayed for more than a year and may never be released. Anyway, SiN does provide some of the best levels in the 90's shooters, i think the level design is only a step behind that of Half-Life, which in my opinion, is the very best to this day when it comes to "environmental storytelling", where the corridors of Black Mesa tell their own story and help you fill the gaps and make your own conclusions, and form theories about the bigger picture of what is happening.
SiN still follows a more traditional level formula for the time, where the levels involve some backtracking and switch hunting, but they are great because they feel natural, like existing places that make the world believable, which was still at the time a very fresh concept compared to the abstract levels of Doom, Quake and Unreal. That being said, all these games are great and i love them to death.
PS: Here's something curios... i've noticed near the end of SiN, where a mutant breaks through the wall, Ross got the Mancini mutant, check it out:
But in my game, i get the big second act boss coming through the wall. How so? Is there different versions? Is it a difficulty thing? I played on normal difficulty so... did Ross play on easy? 
