It's not difficult to do it yourseldf, but OH and I did opt to go through an agency because we had a lot of claims to process, and we simply didn't have the time to keep track of them, send reminders etc. However - the company we used (generally sucessfully) are in receivership so we still have few to do, which we'll do ourselves.
It helps your case if you have account numbers etc as the card issuers will want to look at your original application form to see whether you ticked the PPI slot, did it over the phone or face to face, etc.
There have been several people on here who've reported success in getting money back without the claim companies.