If you make the reader work too hard to understand your character's speech, it is possible that they will get frustrated and stop reading the book.
Recently I checked out The Lady of the Loch by Elizabeth Ann Sarborough. I've read several collaborations that she has done with Anne McCaffrey so I felt fairly confident that I would enjoy one of her solo efforts.
The first chapter was very intriquing. The story is set in Scotland. It opens with a dead woman laid out for burial, then a ritual of asking her to identify her killer is performed. Which she does and so the story is off and running.
Until I got to page ten and the dialogue with the tinklers. I've taken out the non-speaking parts of this section:
Hangin's mak me unaisy jist noo.
Unless there's hawkin' amang the crowd or idle coin tae be freed frae careless pockets. I've nae use for a hangin'
I saw me fither hanget. I've nae wish tae see anither.
Mon, ye stood by me tae ca' the King's man when them ithers would hae hangit me. Will ye and yer young friend nae come wi' us?
And that's where I stopped reading. I won't finish reading the book. Fortunately, I borrowed this from the library instead of buying it from the bookstore, otherwise I would be supremely pissed at wasting US$8.
Some phonetic spelling to give the flavor of the speech is okay. Blowed if I know what "hawkin' amang the crowd" or "tae ca' the King's man" means. I'm not going to struggle for 250 more pages trying to figure out what the characters are saying. I've taken English Literature 101 and struggled through reading Beowulf and Chaucer. I'm done with that kind of reading, so back to the library it goes.