I have the Pontiac Montana, same vehicle only in Pontiac trim. 3.4l V6, auto, air, CC, rear air, and some other etc's. It has had some problems, but overall it has been a pretty good vehicle. I bought it with 50,011 miles back in 01 and it now has 198k miles and still get's fairly good gas mileage. 20-22 city, 25-27 hwy. I run a steady diet of preimum fuel in it and change the oil almost religiously between 3 and 4k miles.
Beyond general maintanence, I have only had to do the following:
Alternator (of course, it's a GM)
Intake manifold gasket (supposedly a problem with these motors) only once
waterpump only once
This weekend, I am actually finishing some preventitive maintanence, because I feel it is time to do these things. I prefer to be anal about my vehicles because I need them to stay on the road.
Rear shocks
Front struts, bearings (these were needed!), tie rods, lower control arms. I decided to do the arms because the front bushings were about 20% separated and the ball joints were tired. New ones from GM were only $130/ea so it was a no brainer for me.
Radiator, as it developed a crack in the end cap about a month ago, radiator hoses and low coolant sensor as it was giving me a false light on my dash. Found out it was that orange coolant sh** GM loves so much caked all over it, but I replaced it anyway.
Only other problem I can think of is when the cabin air filter gets clogged it makes the AC sweat into the carpet. This causes the water to eventually puddle into these areas right below the door under the trim. It wouldn't be a big deal if the GM engineers hadn't run the wiring harness there. I had to replace about 10 corroded fcty splices on both sides. That made my rear air not blow cold. Change that filter every so often and it shouldn't be a problem.
Check to see if it has had the power door recall and the corrected transmission program in the computer.
HTH