We bought our '01 Sienna new in June of '02. No problems with it at all. I tested several other minivans and was astonished at the power of the 3.0L V6 in this car and that's the principal reason I bought it. It also impressed as being quiet, comfortable and solid. After 62K miles, it's just as quiet as the day we bought it. We've gotten 26mpg on many long trips, which seems good for a van. Excellent features and sound system. The driver's side electric mirror has become a little balky and the power door needs a minor adjustment. NO other problems. Our experience with this has led us to buy two more Toyotas since, which are also great cars.
It's very, very quiet. Standing near it you can hardly tell it's running until the fan kicks in. Far less engine noise than a neighbor's much newer Chrysler minivan. Extremely comfortable seating. Like the adjustable rear seats for adjusting passenger comfort/luggage room.
Tires wear more quickly than I like. I get, at most, 40K miles. Significant understeer. Both acceptable for a 3900lb minivan but improvement would be nice. Heat is a bit slow to come on and, if the A/C could be disabled in defroster mode, that would be nice.