It's pretty annoying if it's excessively bad. But a word or two isn't so harmful. Something like "We need moar good grammar!" isn't overly nerve-wracking to me. In fact I talk like that sometimes, but I would never go so far as to not capitalize my I's on purpose or anything. I despise purposeful grammatical errors if it's an entire sentence and there's absolutely no reason for it.
Once in a while, the words I'm okay with are Moar, Leik, Hai and a few others... but that's about it. Otherwise it's most likely a complete failure in spell-checking skills if I've said something weird.