Vitamin E is an important antioxidant, due to its ability to enter fatty tissue and prevent cell damage there. When taken with food, it also prevents unsaturated fats from becoming oxidized into less healthy forms. It can even be used as a preservative, to prevent rancidity in oils during storage.

While 30IU will prevent diseases of deficiency, doses of 400-800 IU have been studied in adult humans and appear to have only beneficial effects, especially when combined with moderate doses (e.g., 500mg) of Vitamin C. Fat-soluble vitamins like E can build up in fatty tissue over time, leading to gradual toxicity, but 400-800 IU daily will stabilize at a level beneath the danger zone.

Generally speaking, it is best to take 100% d-alpha tocopherol or d-alpha-tocopheryl. I will explain why below:

Vitamin E is found naturally in eight forms: There are the tocopherols and the tocotrienols, each of which comes in alpha, beta, gamma, and delta forms. Alpha-tocopherol is the most bioavailable. There is some belief that the other forms, especially gamma-tocopherol, have special activities which may be valuable, but the verdict is not yet in.
The tocopheryl is a close chemical relative of the tocopherol, and merely indicates that the tocopherol has been bonded to an organic acid to form an acetate or succinate. These forms are more stable and fully acceptable as vitamins.
In nature, only the dextro form of vitamin E exists, which is denoted as d-alpha, d-beta, and so forth. However, when vitamin E was first synthesized, it was created as a racemate, meaning that it had both d-alpha and its mirror-image, l-alpha. No one really knows what the l- form does, but it appears to be inactive and quickly excreted. Cheaper vitamin E pills may contain "dl-alpha." They're not worth the money, though, because at best they will be half junk.
It gets worse, though, because the synthesis was later changed such that there are eight forms of alpha-tocopherol created: RRR (the d form), SSS (the l form), and also RSS, SRS, etc. Only RRR is useful. But they were allowed to keep calling this dl-alpha, meaning that chemists who read the name and assume that it's half as active are still being scammed; it's only 12.5% potency. Curse the FDA! Curse greedy vitamin makers who are debasing the profession!

Although I usually scoff at ads implying that the natural form of a chemical is somehow purer or morally superior to a synthetic one, in this case, damn it, they're right. Good vitamin E can be made in the lab, but if you want to be sure look for a product that's all-natural, or at least one that declares on the label that it's d-alpha-tocopherol or d-alpha tocopheryl [something].

Seriously, taking vitamin E appears to be a no-loss proposition. My generation is going to live to be 140 or something absurd like that; I have that much faith in medical technology. I don't have faith that they'll be able to get us there lucid, smart, or possessing any capacity to enjoy life. That's up to us. Get some sleep, exercise, and take your damn vitamins.