I have one for my PT-1418, and I've used it to cut quite a few stumps. It's probably my least favorite attachment, to be honest. It will do the job, but it takes quite a bit of time. It slings all the debris back at the operator and the tractor, which I really don't like.
That said, it may not be as fast as I'd expect, but it's a heckuva lot faster than cutting a stump by hand. It's also quite good for cutting mini-trenches, as someone else had mentioned. If you want to bury 12V landscape lighting wire or sprinkler hose, etc., it'll cut a trench in dirt in a hurry.
I haven't tried it, but I'll bet this thing would cut a slice in asphalt as well. Might even cut a groove in concrete (but I'll bet you'd have to change the blade after that!)
I got mine with my old used machine, so in my humble opinion, a small stump grinder is better than no stump grinder.
HTH,
Dave