I'll answer for myself. Yes, I've handled several informants and several agents. As a handler, I've worked with them in an undercover capacity. I have dealt with relocated witnesses.
You're right, they're not individuals of good character. If they were, they would likely not be any good to us. They're individuals who have immersed themselves, over a lifetime sometimes, with a criminal element. That's where they become useful to us, because when we try to infiltrate a criminal organization, sometimes members of that criminal organization will not trust anybody they haven't known since childhood. Telephone intercepts are not always beneficial, because they guard themselves very much on the phone. Surveillance is sometimes not very effective, because they're very surveillance conscious. They don't trust anybody; they only talk to their circle.
Using these people sometimes becomes a necessary evil, and I say “evil” to the extent that we're having to deal with people who have a criminal past.