My friend's Sienna XLE Limited has this feature...he hates it. Here are two scenarios:
1) Driving in the left lane, semi truck in the right just ahead of the van. The highway curves to the left. The radar picks up the truck as being in front of the car and the car slows down. Same thing happens if the truck is on the left and the highway curves to the right.
2) Somebody passes the van, then moves over "a bit too soon". The van will slow down, even though the car in front is accelerating.

He mentioned that, depending on how close the car or truck is, the van will actually brake fairly hard. There is an adjustment for sensitivity, but he prefers to just keep it off.

2005 Honda Odyssey EX-L RES & NAV - Desert Rock Metallic

2006 Honda Civic Hybrid NAV - Magnetic Pearl

2007 Toyota FJ Cruiser - Black Cherry (First Toyota, Third non-Honda)