Belg,
I have one of the cutters from micromark and I couldn't live without it! It works great on balsa, and on thin basswood too (1/32 inch). I've cut 1/6 balsa into boards with it, and also 1/8 and 3/32 balsa into VERY thin strips. Micromark claims you can put another type of blade in it to cut basswood, but those blades don't fit the tool.
Things I've noticed about it:
Keep a SHARP blade in it.
The blade should be positioned so the back of the blade is straight up-and-down and the point should JUST touch the surface you're cutting on so it'll cut all the way thru the balsa.
Apply even pressure both downward and against the wood.
Go slowly with it.
When you buy balsa look for very straight sheets, no waves in the edges. If you mess up an edge turn the sheet over and cut from the other (finished) edge. If you mess up the other edge use a straight-edge and xacto knife to cut a new edge on the sheet. Also check the sheet to make sure the grain is parallel to the edge. For the record, I buy my balsa in Michael's.
Don't try to use the took to cross-cut against the grain. Trust me.
You'll mess up a couple sheets of balsa getting the knack, after that it'll work great. Hope this helps!