There is a great site on the web containing lots and lots of info on both Brushy Bill and John Miller, dealing with and dissecting the evidence and claims around these men: It makes for a great read a rainy night over a cup of coffee!
Here is a link to another great forum discussion which I highly recommend that you read (although, after entering, the essentials don't appear until halfway down the page). It deals with evidence for and against the survival of Billy the Kid. Great and interesting reading. Please read all, for a complete picture. I highly recommend it!