1) Don't be a class officer if you are just looking to pad your CV. If you do it right, it is a lot of work but worth it. If you are a typical selfish wad, you won't do your class, yourself or your school any good.
2) I'm glad some people qualified their responses with IMHO. They probably aren't class officers and probably haven't applied to residency yet or even know about sacrificing time and effort for the betterment of your class and school.
3) I am a class officer at my school and am also involved in MSGA. We are sometimes the only student voice in a very big machine that would run over our concerns if a few students didn't represent the rest.
4) Yes, if you play your cards right and do a good job you will earn the respect of the administration, your attendings and your fellow classmates. That will take you a long way into optimizing your chances of a successful career.
I can't say for sure whether it will help landing a good residency. The feedback I have received says it will. If your looking to get a highly sought after residency, you will be competing with those who made the grades, high class rank and board scores and had time and energy to round out their med school career with extra activities and community involvement.