Only problems have been peeling on the sterring wheel, which was fixed under warranty, and leather seat wear, which was also fixed. Sun roof leaks (yikes!) No mechanical problems at all -- very fun to drive!
I am on my second Grand Prix (corporate leases) and I will say that I am happy with the car in general - not thrilled or anything, but it's a great basic car as long as you only drive new every couple of years. My 2006 base model is much improved over my 2004 base model. The 2004 started to have severe transmission and throttle problems at about 28k miles. The throttle problem was fixed with some tweaking to the injectors, but the transmission had to be completely rebuilt t about 36k miles - under warranty thankfully. The 2006 has had a chronic problem in the steering that makes horrible noises at low speed turns. I've had it in the shop three times and they can't figure out what it is, but have rebalanced the wheels, replaced the wheel mounts and repacked the bearings. It does not affect performance, just makes an awful noise. Overall, both cars were comfortable for both short and long drives. the trunk is ample, the backseat is comfortable as well. Mileage was better than expected at 25 mpg on the highway and about 20 around my hilly town. The base engine is peppy enough, no complaints there and the handling is every good. Sometimes I feel like the braking could be a little stronger.
I bought my 2007 pontiac G5 about 2 months ago and I absolutely love it! At first i was a little skeptical about buying a pontiac but im a believer now. the ride is smooth, the G5 is extra sporty to be priced as it is and the Fuel Economy is top notch!!! i would highly recommend the G5 to anyone wanting to save $$$ on fuel costs and not suffer on style or size.