I was turned off him too, because he talked the talk when he couldn't walk the walk. His behaviour with the press when Cowley was shot I found very distasteful, and found it hard to get past that. I agree very much with gemini's comments about him, and also nebraska's point because I also got that impression.
I didn't think that Purvis was actually presented as a sympathetic character for much of the book -- perhaps a tribute to Mr. Burrough's good honest writing.
It's especially the case when we were told about him being in the top ten of heroes of people surveyed, or something of that kind, contrasted with the picture of his ineptitude and ego.
I did have some sympathy for him for the way he was treated by Hoover, who should have dealt with him after the Little Bohemia debacle and then couldn't later. Also, as Mr Burrough says, he was in over his head and wasn't adequately trained.
He isn't the only guy in the book with an over-active ego. But it will be interesting to see how much of that shows in the film. On paper, he has potentially an interesting character arc (possibly more so than Dillinger) depending on how he is portrayed and I would expet that to show through. The link and tension between the hunter and the hunted is at the heart of things, and I think both men will be portrayed sympathetically because it will make a more interesting film that way.