It worked well initially to encapsulate the crumbling black rubber backing on the carpets.
It had two downsides, though. The first was immediate -- the carpet functionally shrinks dimensionally and some of the snaps are impossible to get snapped down. This was less problematic than it sounds like, since I was able to get some down and the added weight of the flex seal coating keeps the carpets down.
The second downside is why I wouldn't use it again as a "long term" fix -- it started peeling after about 4-5 years, and the peeling was a serious mess. The peeling became evident 3 years ago in the fall when I took the carpets home for the winter. My marina/storage guy even commented on it in the following spring, despite me doing my best to hose/vacuum up as much as possible.
Since I really struggle to spend the money and jump through the hoops to replace the carpets (they look OK on top, I scrub them with laundry detergent and a brush chucked in a drill annually) I did a little triage. I peeled off as much of the flex seal as I could (total chore, about 8 hours in the garage with a wire brush and a shop vac) and then used a spray silicone rubber roof seal (Henry 880 Tropi-Cool Stop Leak 100% Silicone Spray Sealer) as a new backing.
It's held up after 1.5 seasons, but I think I'm finally going to have to deal with the carpets.
Flex Seal is a cheap solution, but it will eventually crap out.