You use it when "something" is impressive, and in some cases complex.

"quite a" means "bigger or more than average". To say "this city has quite a reputation" begs the question "a reputation for what?". So, I could say "this city has quite a reputation for its horrendous air pollution" or "this city has quite a reputation for its active nightlife". "quite a" is neither positive or negative. It just means "more than average".