It's been done - someone here posted a description of a quickie test batch using a 2.5 gallon water jug as the fermenter. He punched a hole in the top for an airlock, used the water to make a wort, and ended up with beer.
Upside: cheap single-use container
Downside: cheap single-use container. Integrity of spigot is crucial.
Go for it!
"Anything worth doing, is worth doing slowly." ~~ Mae West