MacFoil uses straight lines to connect the airfoil data points, NOT true curves. This may be a problem for some airfoils, or for very large chords. Most airfoil data I've looked at has a lot of points around the leading edge, so it draws just fine, but if the data is sparse you'll get a polygon look instead of a smooth curve. If this is a problem, please let me know, and if I get a lot of complaints I'll bite the bullet and implement bezier curves.
The size of the drawing is limited, and as a result the chord is limited to something like 200 inches (less when printing, see below). I suspect that most modelers won't need anything even close to that big.
MacFoil usually prints at the highest resolution the printer will support, up to 600 dots per inch. Because of this, you won't be able to print a really large airfoil on a super high resolution printer: you'll get a message that the drawing is too big. If this occurs, try printing at normal resolution (select "Print..." instead of "Print High Resolution..." from the File menu).
Spar positions are saved in files as percent of chord, even if entered as absolute values. So if you have spars specified with absolute positions, closing the file then opening it again will convert them to percent chord, and they'll stay that way. Similarly, fractions aren't saved as fractions, they're saved as decimal values.
MacFoil is a little ugly under OS X. In particular, the text in the editable boxes is uglier than I'd like. This is a side effect of the tools I used to develop MacFoil, and will be fixed in the future (whenever the tools are fixed). I could have spent long, hard hours making it prettier, but thought my time was better spent making it more functional instead.
The testing under OS X was a bit sparse: if you're running MacFoil under OS X, PLEASE tell me about any bugs you find so I can fix them.