We use Fogbugz to keep tabs on problems experienced by our clients. We provide logins to our clients who then can add new issues and track our progress dealing with them, as well as providing a documented medium to request more info etc. All very fine and dandy.
The only major drawback is when we internally are debugging, we want to add internal notes that we don't want to share with the client ("Tried X, did not work", "Issue not so urgent" etc). These would be comments that are visible only to the user that submitted them (or possibly a group of users).
At the moment, we get around this by running two concurrent tracking systems - one client-facing, one internal only. It works, but there's a lot of information duplication and they get out of sync frequently. If Fogzbugz could do both functions, we'd be a very happy lot..
I'm not sure if this is a feature request or a "how do I" :)