Using "u" and other short forms is acceptable in chat and sms, where you have limits on your time and space. I agree with Goatish in that it really looks terrible outside of sms and chat. If you are writing in a formal/business sense, or even just writing to ask for someone's help, then writing "u" makes you look stupid and even rude (this also applies to wanna, gotta and their ilk).
The general rule with writing is, if you have the time and space, then write it out properly.