I always saw it as a bit of a scam to get lazy people who couldn't be bothered to chase it themselves.
But my mother-in-law used one of the firms you see advertised on TV and received thousands back. From old credit cards, even an old Next account. She's very clued up and nothing really gets past her so that has changed my opinion of the firms.
One sister in theatre got about £10k from an old mortgage years ago too. She'd forgotten about the claim and around 6 months later received confirmation of the money she was owed.
I suppose you could say it's money you absolutely wouldn't have had, so the PPI company getting whatever percentage they ask for is a small price to pay.