I'd say coloradokevin nailed it. Here's my findings down in Florida...
[Type 1] Going by what I've overheard from bystanders as I shoot mine, most people's initial reaction is "I thought those were illegal!" Usually someone else will chime in that they are legal to own, with some form of Government permission, an explanation which is usually erroneous ("Class III License, gotta be a Cop, etc.) Nine times out of ten, those same people who initially asked if they were illegal usually end up saying "That's really cool!" Now, mind you, these are people who are warm or lukewarm to the concept of owning and shooting firearms. These are usually people whom someone else brought them out to go shooting. These newbies will usually ask their buddies why THEY don't own anything like this, and they'll usually grumble something about the Gov't and then saunter away (see "Type 3")
[Type 2] I have actually had a Range Officer at my gun club make a remark to someone else "Why would you want to own such a thing?" I don't usually answer questions that were not directed at me, but in this case I responded "Well, I certainly wouldn't buy something to make my gun LOUDER!" People laughed, and the R.O. made a comment about bringing the Gov't into his life (see "Type 3" below). These guys are the types of people that are Fudds to the core. Most of the time they are completely oblivious to the NFA regulations and requirements. They are seldom worth having a conversation with, but I like to have hope in Americans when it comes to the 2nd.
[Type 3] There is another group of people; these people are regular shooters, but through conversation you find out that they honestly and truly believe that by having registered NFA items, that you have subjected your house to unreasonable searches and seizures at any time, any day of the week. They believe that us NFA owners are on a special list, but that the 10 form 4473s that they filled out this year allow them to maintain their complete anonymity from the Government. I believe these are the people that have teetered on NFA ownership, but either were scared away by someone's BS, or they are using this as an excuse for not taking the time to get in the NFA game.