The skillsaw is my option. I know it fills up, it just takes a little unplug and a good cleaning with a screwdriver.
I know this sounds neanderthal ...but it beats the sawzall hands down.
You can try the blade Glenn suggested also, it just takes a lot longer.
I put a used carbide tipped circular saw blade in, and have at it.. I use a Makita saw. If you can find a wormdrive circular saw, nothing stops them.
Also, some WD-40 cleans up the tar really well..try it.
It's just a tough job. The last option is rent a demo saw...they cut anything, it's what the firefighters use to cut through a roof.
Good luck. Wear your saftey glasses.