I have cut up many a pallet. If you are only going to cut up a couple (or they are very dirty), I use a good reciprocating saw with an aggressive blade. Milwaukee ax blades work very well and last a long time. Lean the pallet up against a wall or tailgate, cut the stringers as close to the runners as you can leaving one or two stringers attached at the bottom of the pallet. Rotate the pallet so the top is now the bottom and finish cutting the remaining stringers. Turn the pallet over and cut the back stringers if it has any. You now will have 2 or three runners. Set them aside to cut later. Done properly, you should never need to bend over except to pick up the cut pieces. Total cutting time is probably about 3-5 min depending on the number and thickness of the stringers.
If you are facing down a truck load of pallets, I use a small chainsaw with a narrow kerf bar and 95vp chain. Stack up several pallets till you get to a comfortable working height. Cut the stringers out of the top pallet and set the runners aside. Replace the top pallet and repeat. This process goes amazingly fast as long as you don’t hit anything.
To cut the runners, a circular saw is the fastest but you really need to watch out for the nails. I usually just use the reciprocating saw and cut the runners in the fork cut outs. At 2 cuts per runner, you are probably looking at about 20-30 seconds per runner.
While there is a good amount of heat in a hardwood pallet, I don’t really think they are worth the time/effort to gather and cut up unless you are really hard pressed for firewood.