So far its an amazing and addictive game, it is a LOT more challenging than the past SimCitys with diseases, much lower income, etc etc. And yes, one time while I was building a city the servers crashed and lost my last hour of work. I was pissed to say the least.
But just remember, its the first day, and there's no way to stress test servers for thousands of players at one time. One of the devs was also saying how in a real city, you aren't in your own bubble, which is why forced multiplayer was implemented. For realism.
And stopping piracy.