Read reviews for the KayakStacker by Yakima Products, Inc. as submitted by your fellow paddlers. All of the reviews are created and written by paddlers like you, so be sure to submit your own review and be part of the community!
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.
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!