The first thing you should know is that I am a fan of inflatable kayaks, kinda the opposite of most people on here. I bought a "used" one of these a little while back after doing a decent amount of research about what a WMK1 is good for. I say "used" because it actually had the typical brand new smell when I pulled it out of the bag along with a normal kink in one of the tubes they have straight from the producer, so it was basically new. The kinks can be annoying, but if you just play around with the tube while it's just a little bit inflated, they can go away pretty easily.
It does everything the Sevylor Rio (the K1's a clone of the Rio) says it can do. It is good for a lazy day going downstream with little or no wind (most inflatables don't act nicely in strong wind). It can take some fairly tough rapids, more than you will initially think it can. It will track pretty well when you are moving and turn quite quickly, even though it doesn't come with a skeg. Just don't stop abruptly when you are moving fast, because it will probably make a complete 180 (duckturn). Many of the features are actually great. The bag that clips in behind the seat holds a decent amount of junk that you'll bring with you, the 350 lbs. capacity is a true 350, the bottom material will need a big mistake to cut or tear it, and the inflatable pad under the seat (on top of the inflatable floor) is really quite comfy.
However, there are some drawbacks. The air port caps for the pontoons actually need to be fully removed to deflate it, so you need to be sure they're on good and snug the next time you inflate it. Stay close to the shore when you start and bring your pump with you; you'll probably have to stop to check the caps and add some air at least once, unless you get it right the first time. The ports for the floor and seat pad are kinda elementary, like big versions of beach ball ports. They won't leak, but you have to pinch them while pushing the air out to deflate them, and that can be a workout if you're by yourself.
And of course, if you don't want to get wet, this is not the boat for you. It's a canoe/kayak hybrid (SOT) and you need to use a kayak paddle, which means you'll splash water all over your legs unless you are a magic paddler.
I can't give this a higher rating than an 8 because there are many better inflatable kayaks, but for the price (new or used) it will be pretty good as a backup/buddy boat or for first time users.