Some guy started harassing me to give him my PSN details so that he could download the Undead Nightmare DLC for RDR - this was while I was playing RDR online. He literally saw no reason why I shouldn't give it to him. Reported his moronic arse without a second thought.
Recently, I had another person send me a message pretending to be a Sony representative and telling me I'll be banned forever. The message looked like it was written by an illiterate child.
I think I reported someone for having a bad username on Xbox Live once. I usually do more of my reporting in MMOs - mainly Champions Online and DCUO, where people tend to make "clones" of preexisting characters from other media, and that's against the ToS.
I know some people are against it, but I didn't want to see the company get sued like City of Heroes did because someone decided to make Iron Man to the T. Now, parodies? They're fine. Hell, I made myself a guy named iRon Dude who was basically a white Iron Man with an afro and sunglasses, used chains, and was mocking Apple and the sheer number of crap for iPods out there. Then I made Dead Poo, who looked kinda like Deadpool but was brown in color, threw "toxic balls" colored orangish-brown, and rode around on what I called his flying frozen poo-sicle. He hates that knockoff who's real popular and has his own games, comics, movies, whatever.