Surprised to see some of the opinions on SPFs in here. They have been my favorite wheel for the past several years so I figured I'd toss my 2 cents in.
The 84b ones could be a tiny bit slippery on really slick surfaces, but without a doubt the fastest wheel unless there's some underdog from ricta or something. I would recommend them on skateparks with that slightly weathered, textured concrete.
The 81b ones on the other hand grip really well, to the point they stick a little on some slides. Almost as fast as well.
The reason I bring all this up is because I moved to a new town and the local is insanely slick, slickest bowl I've ever skated. I tried both spitfire f4 101a and oj elite 101a, and with both of them I would find myself doing unexpected slides mid bowl run that would terrify me and kill my confidence. Really I would have preferred to keep either of those wheels because they could be used in the streets, unlike SPF.
We also get real winters here, have been skating in freezing temps and I do not find the 81b SPF wheels to lose traction in the winter.
Don't mean to contradict anyone's opinions here but I do feel that SPFs are a pretty amazing skatepark wheel. I say that as someone who wants to quit them, so I don't have to use a separate set of wheels in the street. I simply haven't been able to stop using them because they work for me.
I wonder if some of the negative sentiments I've read here are more preference based, since Bones wheels really do have unique slide characteristics. It's quite different than a spitfire 99a, and I think if you are used to that, SPFs might feel alien enough to just turn you off completely.