The Task Force Drywall Keyhole Saw or "jab saw" is inexpensive, modest and exactly what is needed to cut drywall. I've been using one for years now and I've been very pleased. The blade is not so sharp, IT DOESN'T NEED TO BE! Drywall is a very easy thing to cut and I've had problems with jab saws that have double action, razor sharp blades because as soon as you hit something in a wall or ceiling that you don't want to damage it's too late. The Task Force jab saw doesn't have this problem. The blade is sharp enough to cut drywall and that's about it. Also, the blade is flexible so at times when I need to put an angle or bevel on my cut or if I'm in a tight spot I don't have to fight with a rigid blade. When I'm done and the blade is all bent up I simply bend it back.
The Task Force Drywall Keyhole Saw is probably one of the cheapest of its kind you'll find in a hardware store but I truly believe it to be the best.