I don't know of anybody that finished an 80% frame, but I do know some folks that build a shooter from Essex frames. The Essex company says that their frames are mil-spec, and ready for parts.
However, if you visit their web site, or talk to them, you'll find that they advise that some fitting will have to be done, and should be done by a gunsmith.
My experience with Essex was discussed earlier on this thread (it may be in the archives), and the experiences I had with the Essex product have been repeated with another make of frame.
One man I talked to said that when he tried to assemble a working gun from his Essex frame, he had to go through a pile of parts, selecting and rejecting each one until he found one that would fit. He said (after a lot of hemming and hawing) that he had to hand fit all the parts in the frame this way before he had a shooter. I suppose this wouldn't be much of a problem if you had a dozen or so hammers, sears, sear pins, triggers, mainspring housings, and so on, which he had, but if you don't have the inventory of a parts dealer, you will likely find yourself hand fitting some parts, or paying a 'smith to do it.
Now, this is with frames that are advertised as ready for parts installation...in other words, 100% frames. Simple logic suggests that an 80% frame will present these problems, and more.
To answer your question directly, he claimed that he got the one he bought working, and used it for casual competition, then got another one for his son, and repeated the whole experience. He did say that he got both of them working well enough to use in local competition.
I didn't see any trophies in his office, though.
The most common machine work I think you'll need to have done is reducing slide rail thickness, pin hole reaming, mag catch hole enlargement, and maybe mainspring housing area enlargement, or MS housing rail thickness reduction.
The frame I bought a few weeks ago will need slide rail work, MS housing rail work, and some material removed from the walls of the MS housing area in the frame.
Hope this helps somewhat.