Stihl does actually make saw blades for their brush saws. The brush saw is basically an upgraded weed wacker. They do require "handle bars" on the trimmer before you can put the saw blade on it.

That said the limit is something like a 6" tree for it to cut and even then I think you were supposed to do 3" from either side. My trimmer came with the brush blade but I have not used it yet. Instead I mostly use the hardy grass blade. This is a metal 3 toothed blade that cuts through hardy weeds really well. And once one side dulls down you flip it over and use the other side. If you are looking for something to chew through all the nettles, reeds, tiny saplings, or thick grass I would suggest getting one of these blades. That said if you are mostly doing grass I would just stick with the string.

This year I am using a carbide tipped 230mm blade on my brush cutter. I couldn't live without it. Well, I guess I could live without it, but it sure makes brush cutting a LOT faster. It isn't as wonderful as my old blade a friend accidentally destroyed, but still pretty good.

Of course, nylon string is great for short lawn grass and edging. Everything has its place. I do the pasture with the carbide tipped "chipsaw" blade, around the rocks, house, and trees with nylon. Swapping out blades only takes a few minutes.

My advice though- don't use it for any woody stems larger than 2cm diameter. The shock from biting in is too hard on the head of the cutter. And once that goes, it is almost the same price as buying a new one (speaking of- don't forget to pack grease in your cutter head after every 15 hours of cutting!).

But as you said- a hot knife though butter. I feel like a Jedi wielding a lightsaber!