I love our "Eighth stow away seat". It has been very handy for us. I have actually installed a couple of carseats in that seat using the seatbelt, and have got a good install. It is possible especially with a Britax that has a smaller foot on the bottom of the carseat. I love that I can take that seat out very easily and stow it internally inthe van. The Lazy Susan is also a great feature of the Odyssey. You may check that out if you haven't. Here are some pictures of our Odyssey. You can see in these photos we have the eighth seat folded down with makes a great hard surfaced holder for drinks, misc. items (when the vehicle is not moving, of course) In the second photo you can see a Marathon installed rear-facing, and that Eighth seat up. In that particular photo the headrest is all the way down, but it actually rises up pretty far. That seat also has an adjustable recline feature. Very nice..
Darren, mentioned that the Eighth seat is good for boosters and young children, but I often sit back there in between my two babies very comfortably. Unfortunately, it is not safe because the head rest hits me in the upper back ( I am 5' 9"), but it can come in handy if we are stopped, and I need to give them attention. I have actually breastfed between the two captain's chairs in the eighth seat.
I always seem to learn something new about the Odyssey. There are some pretty amazing features on it, including safety. I don't know if the Sienna has these features, but either way they are both awesome vans. YOu can also go here to get more info regarding the Odyssey.
http://www.hondanews.com/catID2059
Another new thing I learned this week is that I don't have to get my oil changed every 3,000 miles. The service manager told me to only come in when my oil life computer says 15%. I had gone in at 3,000 mile interval but it still had a 50% oil life on it. Pretty sweet!! It has a built in computer (like BMW's do) that record and calculate how you are driving, and what kind of conditions you are driving in. So you only need an oil change when it says so. He said Honda is trying to lower Maintenance cost on their newer vehicles. YIPPEE!!