Half Life 2 owners will no doubt know what to expect from the Source engine so far, SiN makes use of much of these for a well rounded experience. One of my favorites of course is the physics of the explosive items, seems like there's always some fun to be had in a giant chain reaction of gas tanks and barrels shooting around and blowing up setting opponents on fire. Of course this isn't to knock the modeling of the city, the buildings, the landscapes and the characters. All of those are performed exceptionally well but I tend to get caught up in the little niche items that tend to effect the gameplay a bit more.
Overall the visual presentation is pretty strong for a Source powered game, making use of the lighting, landscape systems and of course water effects the game is pretty strong in the visual arena with solid delivery of interactive objects, secret spots and tweaks on the general aspects usually seen in Source such as the jetpacks, the gas leaks from the large tanks, debris in things such as spotlights and other items. Given that much of this was done pre-Lost Coast I'd say it works well to use a lot of what source offers especially in the actor models where they strive to try and make use of the facial systems as much as possible.
The audio system in SiN really comes down to the end user, I tend to prefer the multi-channel experience as much as I can so I was able to take in the game very well over the speakers but the game didn't lose all that much when downsampling to 2 channel audio for headphones. I think the ambience and the fast pace help the game in terms of immerging the player into the world especially when trying to play it low key with fast stealth movements without losing insane amounts of health points. Hearing footsteps, cross talk and other items tend to lend to how the area can be played and how bad a situation might be looking at any given point.