I like to look at this question from a few of directions.

First, any product that claims to be of general benefit is automatically suspect. Multivitamins, like any one-size-fits-all product will always fall short of the claims made. This is, of course, a general rule and does not address multivitamin supplements specifically. However, I think it is a good way to approach any product or service (especially one involving health) that makes sweeping claims.

Second, there are studies that look at specific vitamins and or supplements; however, I have found it difficult to locate research regarding multivitamins. My search is by no means exhaustive. What I was able to locate were research articles which find either no positive effects on health for specific vitamins and supplements.

In fact there is a good literature review and meta-analysis (published by The Cochrane Collaboration in 2009) of 67 trials involving 230,000+ test subjects assessing the affects of antioxidant supplements (beta-carotene, vitamin A, vitamin C, vitamin E, and selenium) versus placebo or no intervention on mortality. Some of the test subjects were healthy, some suffered various diseases (gastrointestinal, cardiovascular, neurological, ocular, dermatological, rheumatoid, renal, endocrinological, or unspecified). The basic conclusion: There is "no evidence to support antioxidant supplements to prevent mortality in healthy people or patients with various diseases." Instead, the authors found that vitamin A, beta-carotene, and vitamin E significantly increased mortality in some trials. While the authors conclude further study is required, they caution such research, and marketing should be weary of the potential health risk of these supplements. The other supplements had no significant positive nor negative affect on mortality.

While these results and conclusions are by no means a comprehensive appraisal of all vitamins and supplements, and do not address multivitamins specifically, they do not support claims that such products will benefit those taking them. The study includes comments and criticisms from other researchers in the field and is worth the read if you have the time. [Reference: Antioxidant supplements for prevention of mortality in healthy participants and patients with various diseases (Review) Authors: Bjelakovic G, Nikolova D, Gluud LL, Simonetti RG, Gluud C].

And, finally, as far as consensus on multivitamins, there can be none until there is research weighted for or against benefit or harm. Another meta-study funded by the National Institutes of Health and conducted by the Johns Hopkins Evidence-based Practice Center found "Evidence is insufficient to prove the presence or absence of benefits from use of multivitamin and mineral supplements to prevent cancer and chronic disease." [Reference: The Efficacy and Safety of Multivitamin and Mineral Supplement Use To Prevent Cancer and Chronic Disease in Adults: A Systematic Review for a National Institutes of Health State-of-the-Science Conference, 2006. Authors: 1. Han-Yao Huang, PhD, MPH; 2. Benjamin Caballero, MD, PhD; 3. Stephanie Chang, MD; 4. Anthony J. Alberg, PhD, MPH; 5. Richard D. Semba, MD, MPH; 6. Christine R. Schneyer, MD; 7. Renee F. Wilson, MSc; 8. Ting-Yuan Cheng, MSc; 9. Jason Vassy, MPH; 10. Gregory Prokopowicz, MD, MPH; 11. George J. Barnes II, BA; and 12. Eric B. Bass, MD, MPH].

While there is no consensus, the trend seems to be away from multivitamin supplement efficacy. There have been individual studies and research that show benefits for specific vitamins and other supplements (such as specific positive results for vitamin D). Additional research with differing methodologies needs to be conducted.