I don't think that liquid vitamins are really something that should be considered as more than a fad. Vitamin tablets have been around for decades, and they have done extremely well for millions of people. I'm also just not really sure about the quality control for liquid vitamins, it could just be flavored syrup.
I prefer vitamin tablets because that is what I have always taken. Then again, I may be wrong about liquid tablets. They may be just as effective as vitamin tablets. In the end, I believe it all comes down to user preference and what the consumer likes. I still believe that vitamin tablets are better because the vitamins haven't been mixed in some liquid form.
I prefer to consume my vitamins in the form of tablets because it is much faster and simpler. If an individual chooses to swallow a simple tablet as opposed to a liquid form, it takes a single gulp instead of many. It is perfect for the health conscious person with a busy schedule.
No vitamin supplements do much for you at all. Most of the time you will simply urinate out all of the vitamins before any of them are actually absorbed. That said, things like slow release tablets release slightly more vitamins into the body than liquids, so they get the very slight nod.