The Kayak Stackers perform as promised. I carry 2 boats, one on each side. I agree with the reviews above, though. You'll want to carefully think about your available roof-space and your ability to reach. I have a Forester, and I have no problem reaching up and over to secure them. However, keep in mind that once you have the first boat up, it can be harder to reach around it to secure the other one. Definitely think about what order you mount the boats in!
Rated 8 out of 10 for price to performance ratio. Lost 2 points due to cumbersome design when not in use. You really can't leave them sticking up (well, I guess you could, but they look strange and probably don't help your mileage), and if you fold them down (round bars only, thank you), there's a chance they could work loose and start rubbing/vibrating on your roof causing damage. Yakima Stackers may be well and in some cases the perfect answer. However, if you can't reach them then they are a total waste of money! Don't put them on top of a van unless you are very tall yourself.
Don't let someone sell you these when what you really need is an oversized bar with rollers and saddles of some type. Go to someplace where the employees will work with you and your abilities or lack thereof and get the right set up for what you can actually handle physically!