This game had the potential to be a 10 but two major pitfalls will give it an 8.
For starters, let me talk about the positive sides since it does have many. The game is addictive. The layers upon layers of depth will keep you playing for hours, days, and months, if not years. The replay value of the game is definately a 10.
MADNESS IN THE DETAIL
There are few games that pay so much attention to detail as this one. You can explore your city in the closest Zoom-Level and see all the little details like signs in front of houses, old tires, flowers, and much more. Not only was there attention to detail but a vast amount of it. As your houses/buildings progress through their population stages, they develop into one of several possibilities. In other words, at a population level of X you will have Y houses that might show up, depending also on Z, the level of zoning, and W, the level of income. As you can see, the possibilities start becoming endless.
SO MUCH GAMEPLAY
How do you build a proper road system? Can you properly zone a city (you know rich people don't like living next to dumps). Is your water system costing too much? Should the north side of town be more educated? Where should you put the fire station where it will be most effective.
If you like micro-managing every detail of city-building and management then this game is for you. The game definitely does not lack in the number of variables that affect the outcome of how your city will end up.
SIMREGION
A fantastic addition to Sim City 4 was the concept of regions and urban sprawl. Now you can build your cities in your metropolis that can all be interconnected and even affect each other as if they were almost one gigantic city.
THE DOWNFALL
Let me be blunt: the game is buggy. Patch after patch had to be downloaded to make the game run properly and it still doesn't. Performance is also a major problem on older systems. I would not even attempt it on anything less than a P4 with a decent gfx card.