I've been using a Paddleboy Nemo to transport my Wilderness Systems Tarpon 16 for about 2 years. It's my first cart, so I have nothing to compare it to. It does the job - it is sturdy, and it breaks down for easy storage in my boat.
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.