I think it's alright to type "u" once and a while but, if it is carried too far then, it is just plain moronic.
Example of what I think is acceptable: Yes, I agree with u. Balto does seem to have changed colours throughout the movies.
Example of what I think is rediculous: YEA! I rly feel u when u tlk 'boot dat! L8ter!
As you see, the one above is waaayyy worse than the first!XP
Well, that's my opinion.
