At the end of the day, multivitamins are designed to flood your system with a large excess of what your body can absorb, ergo your body takes what it needs from them. So if you don't have a vitamin rich diet then they will help. If you have a vitamin rich diet they just provide top ups. For most people you just end up pissing out 70/80% of what you put in!
Personally if i get a cold i find Vit C helps, if i'm hungover a multivitamin drink rehydrates, balances electrolytes and tops up and acute deficiencys, so while in whole they are pretty useless they have they're time and place!!