I like them when I don't have much time to read something long and drawn out, and just want a good short story. As many have mentioned, it is frustrating when the oneshot is extremely good and the ending feels like it was rushed or unconvincing. Then you just feel like there should be a continuation and there's just not.
I usually prefer a 5-10 chapter manga - at least then if the storyline is a bit more involved, you have a little more time with the character and usually a more conclusive and fulfilling ending.