PVC is an excellent solution for above ground forming. It does nothing for stiffness so you can only count on the stiffness of the concrete itself but a 12" concrete pier is much stiffer than an 8" pipe. Now that online retailers are selling cut lengths, it's a very economical way to get a very stiff pier with a much cleaner finish than Sonotube. PVC doesn't take paint well but the printing can be removed with solvent for a pretty clean white finish and you can also put a vinyl wrap on it to have whatever finish you like.
For comparison, using our example of a 5 pound horizontal force applied at the top of a 48" high pier:
8" schedule 40 steel pipe deflection = 0.167 arc-sec
8" concrete deflection = 0.91 arc-sec
10" concrete deflection = 0.3708 arc-sec
12" concrete deflection = 0.074 arc-sec
5' of 12" schedule 40 PVC is about $110. Dig a 12" hole 5' deep, pour concrete to ground level with four 3/8" vertical rebar, then slip the PVC over the rebar cage, brace it thoroughly, pour the rest of the concrete, embed your bolts, and for the cost of the pipe, rebar, bolts, and about 16 bags of concrete (about $200 total), you have a pier 5 times stiffer than a $2,000 store bought pier bolted to a 4,000 pound block of concrete. Calculate your actual savings, divide by 2, send me a High Point gift card for that amount and then Bob is your uncle.
All sorts of working solutions and none are wrong. But, when we look at properties of materials and soils and get a handle on how much deflection is acceptable, the stiffest, least moving solutions turn out to also be the least expensive.
Someone mentioned curing concrete inside pvc earlier in this thread. Not an issue. The concrete will cure even under water. It will cure slower which is a good thing. we want to retard the curing of the surface as much as we can. Throw a wet towel over the top of your PVC pier and keep it wet for a week. Follow directions on the concrete bag and don't mess up the water content. When the concrete is cured, you will notice a hairline gap between the PVC and the concrete. Not a defect. The concrete simply shrinks a bit and the PVC isn't doing anything for us in terms of stiffness anyway.