I think gap years can be personally fulfilling, and many people seem to enjoy them. Impressive to others though? I just think of it as a hobby not much different to any other (but obviously fairly expensive). Some people have something really interesting to say about their travels, some people have something really interesting to say about computers, or pets, or their sport.
It is only annoying when people talk about what have become fairly generic tourist experiences and expect others to be impressed at their worldliness/experiences/enlightenment/superiority relative to those who have actually being doing something not necessarily more, but just as worthwhile (work, studies, you name it). When in many cases all they have done is spent money on an interest which is actually very popular. Not a bad thing to do, but not especially impressive.
Then again many/most people I know who have done a gap year/travelling aren't superior about it, they just like to talk about their experiences, which are often interesting. Basically, it's fine just don't get all pretentious about it.