The oxygen sensors and catalytic converter were replaced at 80,000 miles. They were NOT replaced again between 80,000 miles and 403,000 miles.
The water pump was replaced at 300,000 as a preventative measure, as it was still in good working condition.
Various seals and gaskets began leaking oil at around 100,000 miles. Replacing seals and gaskets did not stop all of the leaks. Between 200,000 miles and 403,000 miles, began adding quart of oil every 2,000 miles, and we no longer replace the leaking gaskets and seals.
The Sienna was designed for hauling eight people plus a lot of cargo, or five people and a LOT of cargo. It does that job very well. It is a smooth highway cruiser, and makes long weekend trips a pleasure.
Its large size makes it harder to park. Many indoor parking lots are designed for smaller vehicles, so it is sometimes necessary to hunt for a large parking space.
The view to the rear is poor, so we always walked behind the Sienna before backing out of the driveway to make sure no kids were nearby. The newer Sienna's have an option for a rear view camera, which would solve that problem.
The first two rows of seats are comfortable for long trips. The third row is really designed more for kids than adults, and adults would not like riding in the third row for more than an hour or so.
The gas mileage has been good for such a large vehicle. We averaged around 21 mpg for the first hundred thousand miles, and between 100,000 miles and 403,000 miles averaged closer to 22 mpg. About 90% of the miles are highway miles, cruising at around 70 miles per hour.
It will soon become a "backup" vehicle, but we hope to keep it to at least 500,000 miles.