I played the game for two days, and this is my first impression:
Good:
1. Curve roads. This is arguably the most important improvement over SimCity 4. If all you want is to build the "most profitable city" or "most efficient city" you probably don't care about this, but for others this is a very important feature.
2. Performance is good. Although the whole game is now in 3D, I don't really have any performance problem with my Core i7-2600 and GeForce 670, with all settings maximized and 1920x1200 resolution.
3. Agent based simulation. I know this is controversial, but I think, if implemented well, agent based simulation is much better than statistical model. For example, in SimCity 4, the coverage of a police station is basically by its range, but that's absurd as in real world it's more like to be based on a combination of population and range (also other factors such as basic crime rate). This is hard to simulate with statistical model. For example, in real world, if your streets are jammed with cars, it can be difficult for police to arrive at a crime scene, and reduce the efficiency of your police force. It can be very difficult to simulate such behavior with a statistical model.
Interesting:
1. The global market. The price of some "basic materials" is supposed to be fluctuating based on supply and demand. I haven't observed such fluctuation yet, but if implemented well, this could be a very interesting feature.
2. Cooperative play. The ability to play with a friend in the same region seems to be interesting, but I haven't tried that yet.
Bad:
1. Server problem. Of course, this is to be expected for a launch, but some problems are not very good. For example, in some case, if you "claim" a new city, or start a new region, the server may take several minutes to complete this operation, but the client let you to continue. This eventually lead to a client crash, and all you done in the new city is gone. Fortunately, if one may say so, it takes only a few minutes for the client to crash, so you don't lose too much.
2. Minor graphics glitches. Sometimes a building may flash like crazy, or a road may be partly covered by sand. Nothing too big, but annoying.
3. Inter-region simulation problem. In theory, it's possible to provide certain services to other cities in the same region, but for what I've seen, it can't really replace "the real thing." It seems that the designers of the game do not want you to create some "garbage city" which specialize in collecting everyone else's garbage, and that's disappointing.
4. No farms. This is quite an omission, and disappointing, especially considering that they are going for a region based system.
The balance problem, I think, may take some time to sort out. Some buildings are extremely profitable, to the point of ridiculous. For example, a recycling center, while itself quite expensive, can bring in a pretty big profit in a large city, just by selling those recycled materials. Recycling business in the real world is, well, not like this.