Sounds great. But I often have a question about these sorts of things -- in absence of receiving a tangible product or service, how I do know they're actually doing what they say they're doing?

Well, in this case, they seem to give you access to your own database entries so that you can opt back in to any catalog you like.

Now, technically, you can sign up with the Mail Preference Service yourself. This seems to offer some benefits beyond that but it probably depends on how much junk mail you already get and what kind.

Yeah, my SO stumbled on them two days ago and sent the link to me (and a select few others), but I have some serious questions about 'em. They're obviously for profit and seem to just be "aw shucks, we'll do it for you, but if it doesn't take, it ain't our fault". I'd love to be proven wrong, and if I am, I'll be cutting in line to subscribe.

Yeah, I'm sure the only way for such a service to be 100% effective is to actually receive all your mail for you and sort it before delivering it to you. That seems like a big pain in the ass and would introduce delays, etc. I know I can write to companies and ask them to remove me (that MPS link is interesting, stilicho, thanks) but for a nominal fee I'd rather have someone else generate, address, and send all those letters for me. Are they just sitting in a room, cashing everyone's checks and laughing? I guess it's possible. We need someone who's used it to review them, I guess.