Personally, I avoid phonetic spellings and just write down what the character is saying. Broken or cut-off words, weird syntax, colloquial twistings of words, patterns, word choices, abbreviations and word-smooshings like 'gonna' - I use all of these. But I don't spell them phonetically.
Part of the problem is that spelling a word phonetically does not mean that your readers are going to hear it the same way. You write it the way that your ear hears it, but different accents will interpret and hear those phonemes differently, so ultimately you're going to have limited success at best.
It's also a barrier for the reader, because they have to 'tune their ear' to the dialogue/text. Because of this, I'd say that if you're going to do it, do a /lot/ of it rather than just a little, so the reader can get into the sound of it. That's why stories written entirely in phonetic dialect work well - your spend the time to tune your ear early on and you're okay for the rest. But spots here and there are just annoying, because you have to re-adjust your ear all the time and never quite get into the flow or rhythm of it. The accent should be at least passingly familiar to the reader to be easy to read, though, and like lifesharpener says, they're time-limited.
I tend to avoid barriers for readers wherever I can, and dialects are not a challenge I relish, so I run away. Faaaaar away.