First of all, I too am firmly against forced subscriptions, limited activations, etc. If I had known this game required me to install and sign up with Steam, I would never have bought it in the first place. Normally I research these things beforehand, but this was one of the rare occurrences where I shrugged and said "If the original Mount & Blade had no such issues, why should it's expansion pack?"
Now onto the stability issues. My computer isn't particularly new, but it plays the original M&B with no problems and can run Dragon Age. Yet it has trouble with Warband. I have to play on the lowest settings, sometimes there's the odd crash, but worst of all are the load times when booting up or shutting down the game, which take literally five minutes.
And with that, FINALLY, I get to the content of the game. The single player is... well, it's Mount & Blade with a couple of added features. The single-player mode maintains it's highly addictive freeform adventuring, and the new opportunity to become a king may encourage veterans of the previous game to start calling in sick all over again. All that said, if you're buying Warband, it'll probably be for the multiplayer.
The multiplayer is glorious. My first game I was planted on the Nord team. My bearded friends were charging like maniacs and getting picked off by cavalry. I gave orders to form a shield wall on the hill over the chat, and wouldn't you know it, they actually listened. A hail of arrows constantly rained down upon our shields. Sometimes a lucky shot would send the occasional man to Valhalla, but we held firm. As the calvary lost all speed advancing up the steep slope, we cut them down. That's when we broke rank and took the infantry and archers head on, our shouts of "For Odin!" filled the valley as we charged down into their disorganized rabble. Within moments they fled in every direction, to be cut down one by one by large angry men wielding axes the size of an oak tree. I can still hear screams...
There's plenty of match types to be found, and in each you earn money through kills, thereby letting you purchase superior equipment. Balance isn't a major issue here, because an advantage in armour provides a disadvantage in speed. One of my favourite moments which demonstrates this, is when a guy wearing nothing but a loincloth (his name might have been Conan) was wielding a massive two-handed orcish looking sword. Six of us surrounded him in a circle. His sword moved at ungodly speeds, and he took us all out Kill-Bill style.