The new Sienna is very nice for driving bunches of small children around efficiently. But the redesign really ruined a lot of the usability/versatility of minivans. The non-removable, non-foldable middle row kills cargo hauling. The width of the back gate is constricted, now cannot fit objects 4' wide (e.g. sheet of plywood, etc). Shoulder and head room in the back row (due to the new design curvature) is inadequate for adult sized humans and uncomfortable even for middle-school sized children. The console in the front is too big, non-removable, and makes it very difficult to climb to the back (admit it, this is something you've needed to do as a parent).
The fact that Siennas are all hybrid now is a huge advantage over Odyssey- Honda can't seem to do a good job on hybrids.
It's a pleasure to drive and less expensive than the Odyssey. But it (deliberately) lost some of the multi-purpose utility that minivans always had, which might be a big turn off depending on what you expect to use your minivan for. The larger and more versatile Odyssey seems to have retained those advantages, so as usual personal preference is a big part of the choice.
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