Small grammar errors, like for example confusing their there and they're, I don't mind that. I make my fair share myself. Just the other day on another forum I used the word "paordy" when I meant to use "parity" just because they are spelled similarly. If it's kept under check that's not a problem, everyone makes an error now and again. The one that trips me up a lot personally is when to use "its" or "it's".

If the sentence structure is genuinely bad (meaning it looks like a seventh grade english class project), or if there are a lot of spelling errors all over, I won't read it.