My Thule Aero Blade bars and the car's roof line make getting any rack, including the 835 Pro, perfectly lined up a challenge. Strapping a boat that is not very deep in the stern, can be tricky. The straps go around the top of the J rack like most J racks, but, are required to wrap around the end of the base the boat sits on, actually keeping the strap from contacting the boat on the base of the rack, leaving the boat to move. Look at the picture on the box or the Thule site to see what I'm talking about. I ended up getting another set of straps so that I could wrap another strap around the boat and work it down and around the base of the J rack to snug in the stern of my boat, and give extra security up front.
Once on and adjusted, the boat won't go anywhere. The rack comes with two straps and a bow and stern tie down. I added two more straps to keep the boat still, and for more stability considering the short span between racks my car provides. It's a nice rack, but remember, some boats might not fit as well as others.
After ensuring an appropriate amount of tightness and rechecking them every now and then, I've had excellent success in using them. Using 2 systems on each side of the roof easily allows carrying 2 kayaks on top of a typical car roof remembering to keep them far enough apart such that the 2 kayaks aren't "kissing" in the middle after you deploy the J-bars and place the crafts in them.
Transporting a kayak in an almost vertical position, however, particularly if using a wider hull-style kayak, does pose more resistance to side-winds but just use common sense when driving and you can easily detect what kind of forces are impacting your car/kayak. Travel speeds of around 65 mph in calm weather are usually quite comfortable but if there are substantial headwinds you might want to take it more slowly. Conversely, a good tailwind allows you to move along that much more rapidly.
I know that I've concentrated a lot on how secure and safe these units are, and so far so good with no failures. The kit comes with everything you need. It has a pair of good tie-down straps with very thoughtful rubber protectors over the metal buckles which make it safe to throw them over the roof and then back over the kayak without damaging your vehicle. You can store excess strap length within a Velcro pouch in the bottom part of the carrier. Fore and aft ties with ratchet mechanisms are also supplied. I use Thule "quick loops" to attach those to the hood and back hatch which has made life a lot easier than searching for an attachment point underneath a bumper lying on gravel or wet ground. It may also be useful to have a step-stool available both at home and along with you to make the procedure easier since many vehicles will be quite high up to be dealing with these carriers.
I've had no experience so far with any other system so I have no comparison as yet, but I can definitely recommend this carrier when installed as noted above. My rating of a 9 vs a 10 is for the lack of the importance stressed in the instructions on a more secure mounting procedure.
On the car they hold the kayaks very securely. The straps and ratcheting front/rear tie downs are perfect and easy to use/tighten. The folding feature is great, it allows me to get my car in the garage without removing the rack - very convenient. I do wish the holders came with a way to lock them to the bars.
Loading is fairly easy. I'm 5'11" and can load both boats easily after a day of paddling. My wife is about 5'3", and there's no way she could load the boats, so if you're short these probably aren't for you.
All in all for what they are, I couldn't ask for much more.