I do it, but mostly with color. There is a much more limited amount of contrast control available with color materials than there is with b/w ones, and it helps quite a bit. I have done it up to 4x5. It is really the way to print color well, IMHO. It is just as great of a tool with b/w, but seldom as necessary, as there are a million other ways to manipulate b/w prints to get what you want.
The bigger the film, the easier it is IME. The hardest part is registration. That's why the less you have to blow something up (i.e. the larger the film), the easier it is to make it look good.
You can also do masks right on the paper. Aside from having it's "regular" uses, it is one of the main tricks used by Jerry Uelsmann and other composite photo printers.
Can you explain in detail how you do it, and what materials you use for color? How do you do it on paper`??