Yes, I've used Flexseal. Funny, but I just wrote about it in another thread on this forum. I used it on three old neoprene rafts that were suffering from material breakdown, and achieved mixed results. One of the three rafts is still alive. The other two have gone to raft heaven.
It works where it sticks, but the trick is getting the liquid material all over the inside of the tubes. You mix the two parts, squirt the liquid into the raft, and then fully invlate it. You and a couple of friends then lift the raft and keep turning it over and over in every direction until the liquid gels. You'll see it start seeping through the bad spots, sealing from the inside. To that extent, it works.
The problem is that it is almost impossible to get full coverage, and if you stop turning the raft too soon, or if the liquid gels before you get it evenly distributed, it pools and makes thick spots that are semi-stiff and tend to crack. We used it on one raft when it was too warm, so it gelled too quickly. Also, don't deflate the raft for several days after treatment, or you might get adhesions.