I try and make a point of using good grammar 'n all that myself but, I guess to VeggieBoy's dismay, I tend to use colloquialisms a lot. I don't say "y'all" too much, but that doesn't stop me from never using it.
As for other people's grammar, I enjoy being able to read a post with ease, which means it's been at least somewhat properly formatted. I can understand a few errors or so, since no one should really be that perfect, but I hate it when it looks like someone's fingers were in braces and they couldn't reach anything other the numbers and letters.
To put some perspective on it, when I read I try to imagine the person's voice in my head. I don't know why, that's just what I do. When I read a post that's been written without grammar or with a huge amount of 1337, the voice kinda sounds like Meatwad off of ATHF.
Going off of what Aruskano said, I also browse around the Spanish forums, just trying to pick something up from them, tryping to help out (keyword trying). It's kind of confusing when I see shorthand like "no c si deboacer eso y meparece k esta roto". It takes me a little while to figure what they're saying, what with the lack of proper spelling and acentuation. I guess that's my fault for trying to learn off of a message board, though. Plus, it gives me a little more insight on their colloquialisms, I gues.