We have a 2011 Sienna and a 2010 Flex. Are there any specific areas you're concerned about that I can comment on?
In some ways, they're pretty close. For pampering the adults and to a lesser extent the kids, the Flex wins pretty easily. If we're taking 4 adults and 2 kids on a long trip we'll take the Flex if at all possible just due to it being so much quieter and in turn more comfortable for everyone.
For cargo space, the Flex is much better than some with the third row up... for instance you can carry 2 very large suitcases behind the third row standing up. However, it just can't compare to the interior size and flexibility of the Sienna. If we're only taking 6 in the Sienna we know you can fit virtually anything you pick up in the back.
One surprise between the two, is that even without the giant sliding doors the Flex is much easier to load up a kid FF in the third row. It has something do with the height, and actually the rear doors on the Flex are pretty gigantic too so perhaps that helps. If you put the Flex next to the Sienna surprisingly the distance between the front of the front door and the rear of the 2nd row doors is about the same.
So for us the Sienna is the anything is possible vehicle for that 1% of the time while the Flex is the vehicle both of us fight over for the 99% of the time. Neither of us minds the concept of a minivan, it's just that the Flex does some things so much better. The 1 foot thick doors and cutesy 3rd row folding mechanism of the Flex cause its cargo capacity to be out of line with its generous seating capacity otherwise the Sienna might "never" be needed for us.
Jason