I used 2" pipe rather than 3" pipe because that's what it looked like the professional models use. I don't know if 3" would be easier to ride. I've only given it a few tests, but it all seems to work. I will say that riding this thing is not like riding a bike in that it takes a lot more coordination and balance! I think that's a good thing, but it will take some practice to be able to use it like they do in the online videos. I do like the way it feels more than the stand trainer -- more realistic and requires some real concentration.
I've added a video of it in action. You can see that it takes balance and concentration -- i've only done it a few times but am getting better quickly. Ignore the grunting in the background, that wasn't me, someone was doing P90X.