No problems to date.
This minivan has been very reliable and has great value when compared to the competition. The standard features are numerous even on the base model.
The 3.9 engine is very capable even when the van is loaded. But the fuel consumption is about 12l/100km in 50/50 city/highway driving. Although it is a Flexfuel model there are no E85 stations in the Greater Toronto Area. Just as well as the mileage would be even worse.
If I was to find fault it would lie with the seats and the transmission.
The seats are comfortable, but the armrests are short and the rear passengers have no armrests on the base model. That's fine if you are using child seats though.
The van drives more like a car than the other domestic minivans that I drove, but it does rock a little in high winds at highway speeds. The transmission is also a little rough in the city when switching between 1st and 2nd.
My parents have the '03 Pontiac Montana, a virtual sister to the Uplander (except with the 3.4L V6).
It's been absolutely fault free for 65,000 miles now.