I know two people who have claimed for PPI, they both used PPI claims companies but I don't remember who and it was several years ago. My advice to anyone claiming PPI is to DIY it only if you know precisely what amount of money you should receive. If you do, then it's not difficult to fill out a form and lodge the claim.
PPI companies take a big chunk of your cash, but the guy you talk to on the end of the phone is on commission, and if he is experienced and knows his stuff, he'll claim on your behalf for compensation you wouldn't have known you were entitled to. They want their commission, and they know how to play the game.