I think that once you choose to go for a paid signature, you instantly lose a lot of credibility.
With a lot of posts people are going to think: Is he just saying that to get his post count up?
I think this forum itself has lost a lot of credibility by for example, allowing account sales. I don't think I'm losing credibility when I keep an ad in my signature.
If someone who's wearing a paid sig says something you go think if the message is legit or not... something goes wrong there. =) The message of the sig (or the fact it's a paid sig) shouldn't affect the posters message itself.