For one thing, he said the way his worked was that he'd take a mental picture and be able to recall that thing perfectly. So, you could flash (a couple of seconds) him a random page from War and Peace, and he could read it out to you.

However: he didn't automatically know everything on the page. If he wanted to know what out said, he'd still have to take the time to read the page (in his memory).

And there was a downside: if he learned something incorrectly, he couldn't unlearn it. It made things really difficult for him when his friends started getting married and changing their last names... or when people changed their phone numbers, or email addresses. In the latter cases, people he'd known for years who'd had 7 or 8 phone numbers, when he wanted to call them he'd have to mentally work through each number until he got to the last one before he could dial.

He never gave me a strong opinion about whether he was glad to have perfect recall, but he seemed to complain about it more than being grateful for it.