There are two types of power cutters: power shears and nibblers.
Power shears have three blades that do the cutting, two fixed ones on the top and one on the bottom that moves rapidly but with very short strokes up and down between the uppers, It cuts a 1/4" strip out of the metal that coils up as you cut. advantages: fast, easy to control, can cut long strips or straight cuts easily, waste is in one piece for easy disposal, inexpensive for cutting thinner than 18ga. disadvantages: nearly impossible to start in the center of a panel, will not cut tight curves, difficult to cut into an inside corner, expensive if greater than 18ga capacity needed.
Nibbler has a small square or round punch that goes up and down inside a vertical tube that has a slot in the front. The metal is fed into the slot and the punch puches out a small bite of metal with each stroke similar to a paper punch. advantages: can cut complex shapes limited only by the size of the punch, can be started in the center of a panel by drilling a hole the diameter of the anvil (tube), no distortion of the panel being cut. disadvantages: difficult to cut a long straight line, slower, cannot cut tight into an inside corner if it has a round punch, waste is a pile of little metal "nibbles" that are harder to clean up especially if they fall inside a panel such as a door or quarterpanel.
Choose your poison!

 

I have a Mac tools nibbler that spits out thousands of little tiny half moon shaped pieces. They are a pain to clean up(I use a big magnet) It leaves about a 3/32 wide gap where it cuts but it is wicked fast, very accurate and doesn't warp the edge of the cut.

 

Clean-up "tip" put a plastic bag over your magnet, pick up the pieces, turn the bag inside out over them and pull off the magnet. Scrap is all bagged up and ready to toss, magnet is clean.

 

how about small abrasive cut off tool like the muffler shops use?....uses like 3 or 3.5 inch cut-off discs...worked pretty well for me whe I did panel work a while ago...harbor freight has one of these things that looks like a die grinder with a cut off wheel & guard...

 

thanks for the tip Ax, I already do that but it's a good idea for everyone else. I also store my fabricator's magnets in Zip-Loc bags, it keeps chips and grinding residue off of them. That stuff has a way of finding a magnet no matter where you hide them.

Buffalobob, that's the Mac cutoff tool I mentioned earlier. It's awesome.....