I have used a string trimmer on two dogs, and one grown man. Didn't actually contact the man, but it worked none the less.
A good friend just finished a court case involving a string trimmer followed by a Com Toro Zturn. Judge ruled it as self defense.
No doubt ope, while effective, is a far second choice the whats in the glove box. Sometimes It's all you have.
The incident involving the man and the string trimmer happened when I was about 14 years old.