I have the
Tonic 12x12 and
6" guillotine paper cutters. I ordered both of mine from Joann's online. {5/4/12: Both are currently on sale}
I have had both for 3+ years. Both are still very sharp & cut a nice, straight line every time. The 12x12 will cut approx. 3 sheets of heavy cardstock at a time. It also has a finger guard/piece that holds your paper all the way down the edge. Keeps your fingertips attached & helps keep the paper from shifting while you cut!
I keep the smaller trimmer right on my desk for small cuts or "touch-ups" on card edges. The ruled grid lines on it are about 4x6". It is a little bigger than that overall (maybe 5"x8"?)
Like other posters, I have had other cutters/trimmers I tried and was unhappy. I have never been unhappy with the Tonic cutters.