Nothing major has gone wrong with the car, save for CONSTANT need for new brakes. The car is so incredibly under-braked it's a wonder it stops at all.
The transmission started shifting very harshly not long after I purchased the car, but I was told that its normal.
The headlights seem to be aimed directly at the sky, meaning they are WORTHLESS as I find myself only using the fog lights at night. I had them realigned and buffed which worked for a week or two, but they just went right back to the sky.
The traction control works, but I wish it did not as it almost killed me one night when it took over after going through a deep puddle. It almost steered me directly into a tree!
It's a fairly reliable car that starts on even the coldest morning, and the sound system is very clear.
But who's ever idea it was to put the 3100 engine in a car weighing close to 3600lbs should be fired. This car is the most underpowered car I have EVER had the displeasure of driving. It has NO usable power below 3000 RPM's and takes a little over 10 seconds to get to 60MPH. I also find that I rarely get about 17 MPG because of the constant need to feather the gas to keep it up to speed, as it can't stay at 55 for more than a second before it sinks down to 50 then 45 and so on.
I have heard people say that the SE is good on gas being that it is a smaller V6, but I find myself getting maybe 300 miles out of a 20 gallon tank in a good week! (closer to 250 usually)
Also this car has the worst under-steer of any other car I have ever driven, with an even worse turning radius. Usually I have to make a 3 point turn where most cars can easily make a simple U-turn.
BOTTOM LINE: This car is GARBAGE with a cheap interior!
Your car must be defective or something. My Grand Prix is great, and goes 0-60 mph in about 8 seconds. The gas mileage is about 21 in town and I've only had to get the brakes changed once since I bought it in 2004, and now it is 2008.
The gas deal may be different with you though; you might be the first one off the line and be a hard braker.