First, this will most likely be one of the very best cars that you'll ever own. Every aspect of this car is absolutely amazing. The engine is plenty powerful, and the ride is great. The first night that I drove it I didn't want to stop driving. This car has all the features that one could desire.
Now it's true that BMW's aren't the most reliable cars on the planet and when it comes time to fix them it usually wont be cheap. DON'T TAKE A BMW TO THE DEALER, or you're an idiot. Either find a BMW mechanic in your area or do the work yourself, it's very easy. If I, being a 19 year-old college student, can afford to keep this car running, you can too. (continued below)
Remember this car isn't a Honda or Toyota, so don't act like it is one. Also, use websites that carry OEM BMW parts for a discount, you'll be thankful that you did. For example, an oil filter at the dealer = $45. An OEM unit online = $13 plus shipping. That being said, change your oil yourself and save hundreds, very easy task.
The engine!! The silky smooth transmission, the navigation, the million-way power seats, the amazing seat heaters, the legroom in the back, the smooth ride, the handling, the climate control, the attention to detail, and all the features that you probably wont ever discover.
Better quality interior parts. BMW likes to use lots of plastic...