Siege is a ton of fun. At first I ignored the game when it released, thinking it would just another "meh" small team-based FPS game in the vein of the R6 Vegas titles. However, about a week afterward I was reading some discussions on other forums where people were talking about how surprisingly good it is, to the point where I was convinced enough to cough up $60 for it.
Totally worth it. No other game this year has gotten my blood pumping as much as this one. The game focuses on tactical, slow-moving, "check every single corner" kind of gameplay, punctuated by bursts of gunfire, explosions, and other "OH SHIT" moments.
The destructible environments are part of what makes it so great. Different materials in the game react differently and realistically to different weapons. I remember the very first match I played, where I died fairly quickly and spent most of the rest of the game watching someone else's player cam. One of my teammates used the butt of his gun to knock out a hole in a flimsy wooden wall, and then stick the barrel of his gun through the hole to monitor the adjacent room for enemies. It kind of blew my mind.
And you gotta love being the Thermite operator, when you're team is approaching a heavily-reinforced room where you know the hostage is being kept, and you set a breach charge on a reinforced wall, waiting for that section of wall to just completely explode and sometimes you have no idea what it will reveal on the other side. So you just find a good position, aim down sights and hope that your approach toward the objective doesn't totally screw you and the rest of your team over. (Of course, a good team will have a drone operator that remotely scans the room first before breaching and entering).
Another operator I like to play is Sledge, whose main ability is literally just a giant breaching hammer. You feel like a badass just tearing down walls and doors. I've been playing with a friend of mine who always plays a shield class, and we have a buddy system where I will smash down a barricade with my hammer, move aside, and then let him go in with a full-body shield to clear the room. It's great.
I think Siege is a mind game, in a very big way. Sometimes I go around the building and just randomly smash walls and set up explosions to confuse and distract the enemy. My friend and I do this thing where we'll set up a remote breach-charge on one side of a room, and then blow it while smashing another door on the other side of the room at the exact same time. Any kind of strategy you can use to flank the enemy or come at them from an unexpected direction is a good one. It's like a high-stakes game of chess.
This post is already longer than I meant it to be... but one more thing I have to point out is the sound design in this game. The audio fidelity is AMAZING. Guns are loud. Different surfaces make different noises when walking on them. Almost every player stance produces some kind of noise, regardless of how quiet you're trying to be, and sound propagates through walls and rooms the way you would expect it to. One of the scariest things in this game is playing as a defender, holed-up in the basement of a building, and just hearing muffled explosions coming from above you or from beside you. You can really tell how far away a footstep is, or a gunshot is, or an explosion is just by hearing how muffled or how distant it is. You REALLY need to play with a good pair of headphones, with the volume cranked up. You can hear *everything*.
Anyway, to answer OP's questions:
The game is very much a multiplayer-focused title. There is no singleplayer campaign. You can play Terrorist Hunt missions by yourself, but that's about it.
Is the game more fun with friends? Absolutely; but then again, what multiplayer game isn't?
Do you have to be with a group of friends to actually succeed? Eh, not really. I'm actually kind of impressed at how good some of the team communication is on pub servers. It's good to stick together with your team, but you can lone-wolf it too and sometimes do fairly well.
If you have a few minutes to spare, this review articulates what makes the game so good better than I can:
https://www.youtube.com/watch?v=b8AQtI4iTD0
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