jsmoriss: The Toyo M-55s can be siped deeply if you're not pulling a heavy trailer, so I set the depth of the blades to a little less than 3/8" (12/32"). People that haul trailers may opt for 1/4" instead and sipe the tires again once the first sipes are nearly worn off.

It'll take a good 30-45 mins to sipe each tire. It also takes some effort to push the heated knife through the rubber. My plan is to take it easy and sipe one tire per day. :-) The heated knife kit should come with a dozen blades or so, and you may need to change the blade each time you use the knife. They bend back a bit (so you loose a bit of depth) and seem to become brittle after they cool down.