'99 to early '04 models were the worst for Honda. My '01 lost its first transmission at 70k, the second at 154k, and the third at 193k. The fourth has about 45k on it, and I'm hoping Maxlife does better than the Honda fluid did in the first three.
Honda implemented a re-designed tranny during the '04 model year, and they got a bit better.
I agree that the Toyota Sienna's transmission probably stinks the least among FWD minivans, but none of them are great. The truth is, nobody's really figured out how to make a transmission strong enough to move a large FWD vehicle.
Aside from the transmission, mine hasn't had much else go wrong with it. We've had to replace wear-out parts here & there (seals, gaskets, shocks & struts), and the timing belt, tensioner, water pump etc. at 105k and 210k.
If you decide to buy an Odyssey with over 100k, walk away from any without service records for the timing belt and a transmission rebuild. And start saving your money for both in the future. My favorite independent Honda/Acura shop charges about $1000 for the full timing belt service, and about $2500 for a transmission rebuild.