I haven't been so excited while playing an First Person Shooter in a long time.
This is because unlike others games in the market offering lots of niche features, Insurgency excels because it has fantastic gameplay that is both tactical and intense. Insurgency feels more complex than it actually is thanks to some well thought out map design and modes to boot, in both Player vs. Player and Co-operative modes.
There is never a moment where I have thought the game was unbalanced and that's thanks to the Classes and Supply systems. Squads (usually 2 per team, consisiting of around 8 players) consists of one of each class, with the exception of sometimes 2 Riflemen/Fighters (straight up assault rifles, pistols and grenades), so you won't get a whole team of Sniper classes not playing the objectives or Demolition classes constantly RPG-ing from afar.
Weapons feel lethal as it takes about 1-2 body shots to take someone down (3+ if your lucky, usually when you get shot in the legs) and explosives are capable of eliminating an entire team if everyone bunches up (a hard lesson you will learn in Co-op where an Insurgent sees 2 or more of you and takes you all down with one RPG shot, so don't bunch up!).
To compliment all this is some great sound design and soundtracks. Guns don't sound limp or synthesized, they are realistic and sound like they can do some real damage. The same goes for explosives and bullets pinging off of nearby walls.
This is also one of the best looking games running on the Source engine, looking as good if not better than Counter Strike: Global Offensive especially when it comes to weapon models. Great use of lighting for both day and night time scenarios. Only thing I would fault is the character models, which aren't at all bad but could do with some more work as the textures on them look a little flat.
There is also mod support through the Steam Workshop, which allows you to make new guns skins/animations/sound packs, levels, characters skins/sound packs and so on which is a great bonus. The game is also regularly updated too with bug fixes and free content such as new maps and modes and more.
To summarise: For me, Insurgency gets a lot of things right and I really can't fault it, it's a great package at a great price (I bought it half price on a free weekend, but at full price it's still very much worth it!).
Highly Recommended! Even better with friends!