I've gotten pretty far into Sim City, and the only advice I can give prospective buyers is to avoid this game for as long as possible. If you've been seriously considering it, just don't. At the very least wait a while, it's not ready for prime time. A lot of the mechanics in the game are either broken or make absolutely no sense. Here are just a few of the problems I encounter on a regular basis, some of which were also mentioned by Joe.
Traffic at intersections with stop lights is absolutely terrible, to the point where it simply doesn't work. Users have had to invent creative solutions in street layout to either minimize or avoid the use of stoplights altogether, which is ridiculous. Once you reach a certain population this can completely kill your city.
Path finding in the game is stupid, which further contributes to the traffic problem. Vehicles seem to take the shortest path (in terms of distance to destination) no matter what, even if there's a colossal traffic jam in the way and there are many other open routes available. Since practically everything in the game is dependent on this system (cars, public transit, trash, fire, ambulances, tourism, etc)... ya, things don't work so well.
Public services seem to randomly stop working, more specifically trash and recycling pickup.
Rail cars don't work, even on a simple loop, they're simply broken. But at least Maxis has acknowledged this one.
Oh ya, you can lose a city or even an entire region due to some sort of critical (and seemingly prevalent) server syncing bug. Yes, you could lose all your progress with no chance for recovery. Why? How? No one seems to know. It's the "City unable to process" error, and there's a 220+ page forum filled with users who have encountered this problem. Unfortunately I'm one of them. Luckily my city was able to recover after about a week of inaccessibility.
http://answers.ea.com/t5/Miscellaneous-Issues/Info-request-Cannot-process-this-city-at-this-time/td-p/731232/highlight/false
Tourism is broken, largely due to traffic problems. Tourists won't visit your city if you have heavy traffic around tourist destinations. So ya, path finding again. Also, tourist numbers seem to drop off a cliff and never recover for no apparent reason, even in the absence of heavy traffic. Maxis has supposedly addressed this specific problem in their latest 2.0 patch, but I haven't noticed a difference.
Intercity play, the core gameplay mechanic in Sim City, needs some serious tweaking. Lets just call this broken for now too. The idea is that you can share resources between cities so you don't necessarily need duplicate services in a region that can take a significant toll on your city budget, such as a university. The thing is at a certain point it seems like every city needs a copy of every service in order to progress, and certain services don't seem to contribute to the region in the way they're supposed to... such as universities. Also, money transfers between cities can take a ridiculously long time to go through. I waited almost a week once before another city in my region received a transfer I made.
Overall Sim City has been a bit of a disappointment, as you might imagine. There are tons of other critical, practically game breaking bugs out there, but I'm tired of typing. It's just sad because this game really seems like it has the potential to be great. For the first couple hours the experience is honestly fantastic. There's nothing like building up a city and watching it work. But then it breaks, and the reality of the broken features and mechanics sets in. There's no better way to sum up Sim City, the more you play the more frustrated you become.