Here are some pictures and a description of the Thule Slipstream
Kayak rack and loading system.
Basically, it is an all in one solution and contains the saddles, roller, straps, etc. The best way to describe it is that it operates on a "sled". The "sled" attaches to your crossbars and can be slid back for loading a kayak using the rollers. Once the kayak is on the roof, you slide it forward to get the kayak over the roof for travel.
Here is a picture of it in the driving/travel position.
Here is a picture of it slid back to the rear of the van for loading a kayak.
Here is another picture of it slid back for loading. You can see that the rollers extend past the rear of the roof.
It is very easy for two people to load/unload a kayak. It is also very easy for one person to get one end of the kayak into position on the roller.
I haven't had enough time to figure out the best way to load/unload the kayaks by myself yet. However, it shouldn't be difficult. Once the end of the kayak is on the roller, I should be able to stand on the roof and pull it up. I tried pushing it up from the ground by myself, but I'm too short to push it far enough past the pivot point. Besides, this is probably not a good method since you would have to trust that the kayak falls into the proper position.
We are definitely happy with this setup. For two kayaks, we use the slipstream to get the first kayak on the roof, then just transfer it to the second rack. The only downsides to the slipstream that I can think of are as follows: Since it sits on the "sled", it is several inches higher than if attached directly to the crossbars. It is also probably not as sturdy off road, although I don't believe it will be a problem either. Finally, it was not cheap, it costs about $300.
I will post again if/when I master loading/unloading the kayaks by myself.