It's way more vitamin C than you will ever need. Though, it also has several B vitamins in it, and B vitamins are good for boosting energy. For the vitamin C, you can get your daily requirements with a glass of orange juice, which is 1/10th of what EmergenC contains. You wouldn't get much B vitamins from OJ though.
It has some sugar in it. I don't think it makes a difference if you use this or take a vitamin pill. If you think you aren't getting quite enough vitamins from your diet, and are more likely to remember to drink this stuff as take a pill, nothing jumps out as bad about it. Not what it's hyped up to be, and you won't need to drink it 3 or 4 times a day as the site recommends...one would be enough, but nothing harmful there either.
If it tastes good, it's better than soda or those fruit juices with nothing but sugar and minimal nutrients.