It's excellent. I'd have never thought Aranspaffsky had this in him; it's a very patient, very well-paced character study. There were only a couple of moments where his direction style was obvious (I'll not spoil anything). It's got that rare thing of being terribly sad as well as charming and funny (without ever dipping into Wes Anderson-esque absurdity), and a lot of that comes from Rourke. He's utterly convincing, and (although this maybe isn't saying too much) it's the performance of a lifetime: the contrast between how he conducts himself in the ring, all imposing and brash, and out of it.
Lots of people who've watched wrestling in the past will have read Mick Foley's book (among others) or have seen previous docs about The Business, so there'll not be much new there for them in terms of its portrayal here (except that it's really good to see actual underground-circuit wrestlers used in speaking roles); it'll be a real treat for people unfamiliar with it, though ('blading,' talking through the setup of the match, etc). And the in-ring scenes (there's not many) are brutal.