Grammar Girl debunks the myth that it’s wrong to end a sentence in a preposition, but she’s not always cool with it:
When you could leave off the preposition and it wouldn’t change the meaning, you should leave it off.
Where are you at?
She much prefers:
Where are you?
Hey, I’d like to stand up for “Where are you at?” As I see it, the trailing “at” doesn’t affect the denotation of the sentence, but it does affect the connotation, so it’s not truly extraneous.
Of course, “Where are you at?” is not idiomatic to formal writing, so don’t use it there. But what about casual writing or speech? In the right context, the slightly wordier phrase offers a shade of meaning that’s not so easy to coax out of the shorter one. Notice that there are only three places the stress can land in “Where are you?” and each implies a different sentiment. These interpretations are subjective, of course, but here’s what I hear:
Where are you? (Anxiety or concern.) How did you get lost?
Where are you? (Impatience or loneliness.) I’ve been waiting for so long.
Where are you? (Disapproval or disappointment.) Everyone else is here already.
Adding a preposition at the end gives us a new place to put the stress:
Where are you at? (Casual curiosity.) I’ll meet you there, wherever.
It sounds a bit slangy, and that’s good if a casual attitude is what you want to convey.
Like succinctness? Then why not say “Where are you at?”, setting the right tone in four words, and avoiding disaster:
“Where are you? I mean, don’t get me wrong, I haven’t been waiting too too long, I’m just asking so I know where to meet you, looking forward to it, you know.”
“So you’re pissed?”
“No, not at all, sweety. Oh, I wish I had said ‘Where are you at?'”
See also: Omit Needless Words?