How do you feel about people who like to act like they're smarter than everyone else even though they never really accomplished anything with their life? Are they trying to make up for something they lack to feel better about themselves?

I'd say it's extremely subjective. In most cases I've seen, it's about image. Yes, making up for something they lack. On the other hand, sometimes you have the arrogant smart people, who ARE smarter than most, but are pretty crappy in their arrogance.

Achievements are.. Driven by lots of factors. I may be very smart, but have a low drive or motivation and need to find my catalyst to make things happen. With some people, this doesn't happen in their lifetime. Personally, I wouldn't judge them by their accomplishments. I usually try to get to the person to see why they act all holier-than-thou, then proceed to draw my own conclusions.