I would first like to point out that what you eat is of the most importance. However, vitamins and mineral supplements are not worthless. They are meant to "supplement" a healthy diet. There are certain people that may benefit from them more than others. However, I would venture to say that over 95% of people could see some benefit from a proper vitamin regimen since they really don't have a truly healthy eating plan. Surveys have shown that 90% of people think that they eat healthy, but in reality we know this number is far from the truth.
Sunny D is clearly not a good source of vitamin C. This juice is high in calories and has hardly no nutritional value at all. The vitamin c in it is no different than the vitamin c you would take in a supplement since the juice is simply enhanced with the vitamin c.
For those who say eat 3 oranges a day, this is also poor advice. Oranges aren't even the best source of vitamin C. Other foods such as broccoli, cauliflower, red bell peppers, papaya, strawberries, and kale to name a few are better sources of vitamin c. Variety is an important part of a healthy eating plan. You need to get adequate amounts of other vitamins,minerals, antioxidants, etc. in addition to the vitamin c. I am not saying oranges are bad for you.
On to my next point. It isn't as simple as just talking about whether or not you should take a vitamin. Many multi-vitamins contain micro amount of nutrients in crappy forms that aren't absorbed very well. When it comes to getting effective vitamins and minerals, the form it comes in is often very important for getting proper absorption and effectiveness. Of course, this depends on the particular vitamin and mineral in discussion.
Glad to see someone else talking about how magnesium deficient we are as a population. I am a huge proponent of magnesium supplementation since most people are deficient due the use of fertilizers, poor magnesium quality ground, poor food choices, etc. When it comes to oral magnesium supplementation, chelated forms of magnesium are best. Some examples are magnesium glycinate, magnesium taurate, magnesium orotate, magnesium lysinate, magnesium fumarate, etc. Like mentioned earlier in this thread, I also use magnesium oil which is absorbed topically through your skin. Epsom salt baths are another way you can get a lot of magnesium into your body transdermally through your skin.
I could go on for hours and probably days on the topic of discussion here, but I will stop at this for now.