I think that if they have faith and believe that this is what they should be doing, then it's not a problem for me. They aren't hurting anyone and hopefully they will help someone along the way. Personally, I don't talk to them because I have my own religious beliefs and allow them to grow from my discussions with friends and family rather than strangers who have one set of ideas that they are "pushing".
I do think that having these young men live out in the world either before or during their college careers may be an excellent way for them to see the world and be more motivated when they return to their education.
I respect the steps they are taking towards their faith. I respect the commitment it must take to go on a mission in a strange place with strange people. If I have time, I will listen to what they have to say. I enjoy learning about other religions even though I do not believe in them. Usually, I do not have the time to listen. I try to be as polite as I can.
Certainly much is required of Mormon missionaries. I am suspicious of proselytizers in general, but as the previous post says, one should be courteous in dealing with them, as they are in a difficult situation. But I tend to think of religion as an intensely personal thing, and thus don't want to discuss my religious beliefs with others. I wonder if, given the availability of information about the Mormon faith and others online, missionaries have as important a role these days. Anyone who wants to know about the faith, its tenets, etc. can simply go to a website.
I have two main thoughts. First, I do not want to talk to them because I don't want to have to listen to what they have to say but, at the same time, I don't want to be mean to them. This connects to my second thought. These are young men barely out of high school. I've taught many young men who, within a year or so of graduation, are off being missionaries. Whenever I see a Mormon missionary, I picture those guys that I taught, off somewhere strange, doing their best to live up to the demands of their faith. Therefore, I'm always sympathetic and always polite because I think I understand how hard it must be for them to do what they're doing.