Just came across this, and I think this is really a bug,
If you try to add a user that has been deleted, the response code you get is
Array ( [Code] => 0 [Message] => Success )
But the user is not added to the list of users, he are still on the deleted list, since "subscriberAddWithCustomFields" also update a user info, I can understand why "Success" is kind of a valid answer, but it should have something more useful to the developer so he could tell what happened.
This is indeed a problem. We'll look into getting this fixed, and get back to this post when it has been done.
Glad I can help your service :) After all, I lov it ;)