Rotary cutters are very useful, especially for
making lots of straight cuts.  The blade is round,
and roll through the cut ( a lot like a pizza
cutter).  I couldn't cut fabric without one, that I
can say.  One of the cool new features, is that
they make shaped rotary blades - pinking,
deckle, victorian, zig-zag, and others.  The
designs of decorative scissors - straight as an
arrow!

There are also rotary cutters that are larger, and
have guides.  Here's a picture of a 24 inch
rotary cutter - it'll accept the same decorative
blades (45mm) and can be used on larger sized
paper - including a 12" x 12" paper cut
diagonally!  Both of these rotary cutter require
that you use a cutting mat below the blade.
Rotary Cutters