I'm sure we've all read books where the dialogue of certain characters is spelled phonetically how they speak, e.g. dropping 'g's from words ending in '-ing'. That's okay. What's not okay is when every word a character speaks is spelled phonetically, or WORST OF ALL, when the whole book is written in such a manner.
If I can't tell what the words are as I'm reading them, I am not going to bother reading the book.