Who pays for window replacements - condo association or owners?

Our condo has been paying for window replacements. I am not sure this has lead to the best use of funds. I was wondering how other associations deal with this issue. Do owners normally pay for this or does the association? We also are at a point where the current windows are a style that is not energy efficient and we may need to go with a different style. So, my last question is, when choosing a company and window to go with what have been some things that have worked and not worked?