It hurts my eyes. Plain and simple. It's almost a physical pain. I understand being in a hurry, having limited space (twitter, text messages on a T9, that sort of thing) but if you have a full keyboard at your arsenal and unlimited space/messages, then there is absolutely no reason to write out this sentence:
I hope you're having a great day.
hope ur having a gr8 day
You had damn well better have only 5 seconds to type if you're sending that message. It makes you look not only stupid, but either 13-16 or 30+ Yes, either so young you're new to the scene or so old that you learned to text with a T9.
In IM or on forums, I'm flexible. If a phrase/group of words is being said a lot, acronyms don't bother me, nor do some abbreviations. Acronyms don't even really bother me in general, it's the mutilation of words, particularly by including numbers in place of letters. That's for license plates, people, not communication. I. DESPISE. THAT.
If I'm using words like 'ur' then I'm either in a crisis or I am typing so furiously on my phone that you're glad you're not in front of me, because I'd be shouting or hitting you with my phone. Or my hand is cramping from using that tiny damn qwerty board and we're trying to have a conversation that is too long for texting.
In short, if you have a real keyboard, use real words. PLEASE. Use them correctly.