I've completed two co-op campaigns without any problems and have had great fun. I was very impressed by how smoothly it works, and on the right difficulty the AI really puts up a good fight even against two people working together.
Some people have encountered bugs, crashes, etc. but these problems aren't universally affecting everyone. As usual with a lot of games, the voices of the minority complaining about problems speak louder than those who have no problems at all, thus creating an overly negative picture.
Still, if you're concerned about encountering bugs, it wouldn't hurt to take Elly's advice and wait for patches to be released.
For me personally, the main pro about this game is the AI on the campaign map. It actually feels like other factions are actively trying to win the game and are capable of doing so - something I never experienced in other TW games. Even your allies are likely to turn against you when you become too much of a threat.
On the other hand, quite a few people disagree with me and don't like how the AI is a lot more ruthless than previous TW games - and there are already mods available to make things easier for players who prefer other factions to be doormats.
As for cons, the singleplayer campaign feels brief compared to games like Empire and Medieval II. This was kind've to be expected though, given the game's setting. Similarly there's less replay value than previous total war games. The clans you can play as pretty much all feel the same with the exception of starting location (which can affect difficulty early on) and a few very minor clan bonuses. The game lacks the unit and construction diversity of games like Rome and Medieval I + II (again, mostly due to the setting).
All in all though I would say it's definitely worth the money.