I had Costco's 10'6" WaveStorm SUP for four days. I took it out twice. The first day, my very first day on a SUP, I could barely stay up on it - it was so jittery - took several nice face plants. But, I did play around on it for a couple of hours either kneeling or sitting. I did catch a few waves but I decided the fin was too small. When I got back to the car, I decided the fin was also too flexible/flimsy.
Came back home and read your threads on stability and balance. Waited a few days, for the muscles I'd hadn't used since who knows when to heal, and went out for my second time. Didn't fall at all - I was amazed, like some of you said in other threads, that my balance would improve so quickly. Tried to catch some waves; the SUP board still didn't work well. I even tried just prone paddling into waves but it would turn and plow and flip. These were very small, mushy waves. I found if I just headed straight in, I could ride them - that was it.
Anyway, took it right back to Costco & they took it back, no questions asked (which was the reason I decided I'd try it - they have an exceptional return policy).
(the paddle I was using didn't break)