Yes the parts are interchangable. I've heard that the jets flow a bit different so if you use Holley jets stick with Holley, Demons then stick with Demons.
Demons also work better with a bit more advance than a similar Holley. I have no idea why, they just do.
You either love them or hate them or love to hate them.
Most folks that I hear complain about them is when it comes to the idle circuit and the fact they run fat. If you remember to throw some more advance in there and the fact they all have four corner idle screws then you should be fine.