No quality issues with the first stacker, but the second one had an issue with the top plastic piece pulling away during transit (didn't fall off though), which caused the kayak to loosen up on top. This unit was sent back to LLBean and replaced for free. Overall, I have been happy with them.
I have used the Stacker a couple of dozen times to haul two boats to spots as much as 12 hours away from home. Attaching the Stackers is pretty easy, but requires some acrobatics when I install them on our minivan. It is a bit of a challenge to stand on something and lean over to thread the nuts onto the bolts and hold the thing in place at the same time. It also takes some time to get two boats strapped down to the thing, as I weave the two excellent included straps through both the Stacker and the factory rack of our vehicle. Hint: Thread the straps before putting the boats on.
Except for a trip to a lake just 10 minutes from our house, I would never use this rack without tying down the front and back also, I had one bad experience (my fault) where my boat was dangling from the side of my car on an Interstate due to my rush to get to the water. These bow/stern tiedowns are not included and Thule offers them, but I bought mine at Walmart for about a quarter the money, and they look the same and have served well.
Due to the weight limit of my factory rack, and the lack of horizontal space on the bars, I don't see any way I could haul more than two boats with the Stacker, even though it is advertised for up to four. Also, the instructions for securing four boats are quite unclear and I have no idea how you could actually execute it, even if your rack could accomodate 200lbs. of boat. You certainly could not use the included straps to attach more than two boats.
The racks fold flat when not in use, so I can leave them installed while traveling without fear of hitting them on something. I use a bungee to hold the yaks in place while getting the straps attached. On our first trip out with the Stacker, the wind moving over the straps across the top of the boats create a horrific wind noise, like that of a wounded wildcat. I lasted 10 minutes before I had to stop and try to silence the ear-splitting noise. After two more stops, I found that connecting the two straps together pretty tightly with a bungee (my thanks to the person who invented them) between suppressed the sound and my wife no longer feared for my sanity.
Overall, the Stacker works as advertised for me, just be aware that loading is going to take some time. Also, take the time to leisurely try securing your boat(s) more than one way to see what works for you and your vehicle. The Thule instructions were not the best configuration for me. A last item. You can tighten the boats down too much. I still have a small dent in one of my plastic boats from my over-achievement on one occasion.