It's a good parts cleaner, but it plays heck with your hands. Truthfully, it's
looking more and more like it's worse then was thought on a health front as far
as that goes. Use it, but do use precautions.
You can take it and still run it in the lawnmower engine after using it as a
parts cleaner. You need to filter it, and a fuel filter does a fine job at
this. A bit slow, but effective. Especially after using something like
screening to get the big chunks out first.
You can sorta revitalize the old gas with one of the carb cleaner additives.
It doesn't totally do the job, but for a lawnmower engine, it does an adequate
Don't do any of this if you've got an expensive wiz bang mower that you dearly
love and insist must run flawlessly at all times btw. Use it on an old push
mower that only has to run most of the time, and only run well enough, and you
don't care about smoke and such. You can do damage with old fuel, especially
if it's been used as a parts cleaner.
As far as killing weeds and such goes, it's not that impressive. It's a
surface burn off, without much root damage. It can also create some
interesting fire problems, as people discover sometimes. If you want to kill
weeds, 24D poisons and such do a far better job, are typically safer to use,
often times cheaper, and you don't run the risk of really wild fines and such.
You can also cut the skunky gasoline into your old oil as you dispose of it.
Let your nose be your guide.
Depending on where you are, consider evaporation for disposal. A pan of it
will evaporate pretty quickly, and will not explode, though it will certainly
burn if ignited. A bit risky, can be exciting, but it's effective. If you've
ever wanted to practice putting gasoline fires out with straight water, it's a
good excuse to learn. :-) Though again, all sorts of attention and fines and
such can result.