I was once in a game where someone would write each and every single dialogue of theirs in an incredibly difficult to read faux-English accent. It made it incredibly difficult to read but this person thought it made their character more credible, more realistic. Maybe there is some creditability behind it, I've been to Newcastle a few times and as much as I love the Geordie accent, ran into one such encounter where I was left standing, “Excuse me?” But that was the once, for the most part, I managed to understand everything else.

Of course, that was at the more negative end of the spectrum where they created one based on an outdated stereotype (or maybe they thought that is how we all spoke in England). I've known several people who knew the line with accents, without it becoming an annoyance to read, where they'd throw in their character's accent in dialogue via the manner of phrases or little nuance that allowed you to get a good grasp on their differences and so forth, and of course I knew those who simply left it in the manner of the route Brovo's mentioned, “X said this in Y accent”.