I have the new Quest. I got it in Oct. of 2011 right when they finished re-designing it. I also had a Sienna that had problems with the airbag, ac and power doors.
I really like it...but it is BIG. I am very used to driving it and love the turning radius which makes it easy to park in tight spaces. I am not sure if it drives as smoothly as the Sienna, but it's no big deal to me. I guess it feels like I am driving more of a truck/suv than a car if that makes sense.
The trunk space is humongous. There is space behind the seats with a very deep well underneath it. I also like how the ird row seats fold flat so you can store a huge amount if you aren't using the third row.
We are a family of five and my dh is 6'5" and my ds is 6'1" and they are both comfortable in it. We do a lot of traveling and have been able to fit all we ever needed to.
I would say the gas mileage is about the same as my Sienna.
One thing I hated was the tires they come with. They were worn out by 20k miles. Even with their tires for life plan...which is a scam anyway, they still needed to be replaced way earlier than I ever did with other cars. I recently put four new tires of my choosing on it and it drives and feels so much better.
We looked at SUV's, but for the same price and gas mileage we opted for a TON more space and are very happy with it.
Hope this helps!