While this may be a needed evil
in places where rapid typing is a good thing
, I dislike people who use it in static media, email, or to make a very bad attempt at being a w4r3z d00d
This has annoyed me so greatly I now have a standard retort for it - 'u' is only 'you' if you are speaking formal Dutch. Otherwise, it's 'je'. This has the advantage of being a true and useful factoid. I think. Dutch speakers, feel free to correct me.