I'll pop an accent here and there, both intentionally and not (when drunk I can't help but go Irish) but for the most part, I find it annoying, especially when people who are doing a Scottish accent are doing a bad Connery, or when doing a British, Austin Powers shows up.
I will however drop a Borat unintentionally, then find it fucking annoying and continue with it, that's the only exception to not doing an annoying accent on purpose.
I'm quick with accents, people who have only heard me talk once (i.e. store clerks, random phone calls, others of that nature,) often ask if I'm from out of town, state or country.
Most are done for fun, but can oh! so easily be turned to evil... I've got this Chinese-hating Japanese guy I'll do every once in a while.
Ebonics is the language of the illiterate and uneducated, so I try not to do it, it can only be offensive imo.