Apply generous helpings of truth, in a calm, regretful manner. Note that FUD#2 is usually heaped high with distraction techniques in order to distract the casual fud-victim from the fact that it is, in fact, FUD. ``What if?'' is the easiest of these, because you just answer the question (e.g. ``What if it breaks? Who's going to fix it?'').
Beware here, though. I have encountered at least one FUDmonger (http://www.halcyon.org/~dick) who uses this distraction method effectively by asking questions which cannot be answered but which implicitly state that your product is evil, bad, or nasty. This is the one case where using counter-spin that attacks an individual may be helpful. For example, ``All reputable sources say that Rails to Trails projects actually reduce crime in a community. The only person who says different is some California consultant who makes his living by going to where these projects are happening and scaring people into hiring him with these ridiculous stories.'' Please note, however, that this only works when you have identified the source of the FUD, and where you have identified that the people who are spreading the FUD are not, in fact, the people who originated it. They are bright, seemingly sincere, and utterly convincing. That is their profession. Thus you must do research to find the source of the FUD, else you're going to be continually blindsided by new FUD from that person that you have no immediate response to. I've seen it in action. It's not pretty. You also must be aiming to convince a larger audience, not the person who ``bought'' the FUD - that person is not going to change his mind, because professional FUDmongers are very effective at what they do.
Attacks on individuals is an effective distraction but can be spun to your advantage. For example, ``I'm not going to use some operating system programmed by pimple-faced geeks with no social life'' is an opportunity to enlighten the guy on the facts (i.e. that the typical Linux programmer has more than ten years' experience in the computer industry, 20% of them have doctorate degrees, etc.). It may be helpful to say ``I wouldn't either. That's why I use Linux, because ...'' This does two things. It makes you appear more credible (since you're actually agreeing with the guy!), and enlightens him as to the true facts.