I built one of the John Smith 20 note organs 6 or 7 years ago. It was a great project, lots of fun to take to shows- I call it my 'smile machine' because everyone that passes it breaks out in big smiles.
Be very cautious of the plans- trust NOTHING until you double check the other parts., Although the John Smith plans have opened a huge hobby, they are really very poor plans. I guess he understands that because he has a note bout it in the plans and on his web page. On the 20 note he gives some dimensions for the feed rollers, and he gives some text about the ratio. They are not the same, if you build it to the drawings the speed is wrong. My organ has lots of extra holes in the case where I had to move things to get them to fit properly. I started out just taking the drawing and making parts, only when I went to put things together did I figure out the problems.
I did not find any errors in the pipe dimensions.
I just ordered the plans for the 26 note and the Midi organ. Again, the plans are just crude hand sketches, and it takes a long time to study and get them figured out- he does include a DVD that helps a lot. I want to build a MIDI organ, probably a 65 key.
Its a great project, lots of fun to show and both satisfying and frustrating to build. There have been a couple other 20 note and one 26 note at Cabin Fever. The 26 note was a beautiful job, sorry I lost the name of the fellow that built it. He was there last year, hope to see it again this year.