I made my girlfriend cum in my mouth once when I was eating her out. I could tell it wasn't pee, because it was sweet and slightly sticky. It was so good, it made me cum, even though I wasn't even touching myself.
Every time I've tried since then though, I just made her lose control of her bladder while cumming, so I got a mouthful of pee. Not totally disgusting, but she was dehydrated, so it was pretty bitter :(