How long have you been using it?
If someone can break into intuit's (mint's parent company) data bases and decode them, I would think they would go after turbotax before mint.
Additionally, your bank/credit card company is probably way less secure than intuit is.
Finally, most people have no problem allowing some broke arse waiter to go into the back of a restaurant with all of their CC information, but are, for some reason, terrified of the internet. It makes very little sense.
Hope this eased your mind.