I have own one for at about 10 years and have transported kayaks and canoes alike with full loads on them. The best thing about cart is that it all comes apart easily with metal push buttons that slide into tubes to connect it together. After you take in apart it should fit easily into hatch on kayak. I am looking to buy another one but I am not sure if they still sell them.
I do have a few issues with it though. First of all, it stinks something awful! I have been told it's the cheap grease that they use. I have sprayed it down with degreaser, and scrubbed it several times and I can't get rid of the smell! If I transport it in the back of my SUV, I have to drive with the windows open! You have to make sure that your kayak is securely strapped to the cart or it will fall off. The hardware on the cart is not stainless, so it rusts very quickly. I will need to replace it this year. With the quick release wheels, different wheel/tire combinations (ballon tires for instance - the current tires don't do well in the soft stuff)would make sense.
For the price, I was expecting a little more durability (and a lot less stench). I think that it is a good concept, but Seattle Sports needs to use a less funky grease, and better hardware.
A boat strap would have been a nice addition. You do have to strap down your boat tight to go any distance - but that's the deal with any cart. Your typical roof rack strap is too long, so a nice "shortie" to match the cart would be good.
And the annoying aspect - the grease they use during factory assembly really stinks bad! I wiped off most of the offending stuff, but it was so potent I was smelling it through the shipping box. It also pervaded my car the first time or to I took it out with me. Note to manufacturer - use any cheap grease that doesn't stink so bad, there has to be one.