Most are, ie, most are like large roller blade wheels of solid rubber. If you rski mostly on nice asphalt, the solid wheels are fine. However, I also have a set with 6in inflatable tires. The larger diameter and the bump absorbing tires allow them to be used on gravel, hard pack dirt, and on crappy roads. For me, if I stay on the bike path around White Rock Lake, the solid tires are fine. However, to get in hills I have to go off into the streets, whose surface is not always so good or consistent. I couldn't do it with the solid wheels -- I'd be thrown to the ground constantly. Details here.
BTW, changing the tires on these things is a real b**tch. My tire levers are not strong enough and I have to go to the bike shop to get better ones. I should send you a wheel; when you need to punish tRicky for misdeeds, lock him in a room and tell him he can't come out until the tube is changed. You will have a weekend free to yourself.