As mentioned in previous responses, there is differing opinion among aquarists on this issue. There is however scientific fact that can't be ignored.
First off, there are two different things: one is 100% pure bacteria, the other is a chemical substance. "Cycle" is the latter, and although I have used it (back in the 1990's) I wouldn't now because there are pure bacteria products available and they are preferable in my opinion. I like keeping chemicals out of my aquaria.
Dr. Timothy Hovanec developed the process to keep bacteria alive in a bottle. Here's a link to his website: DrTims Aquatics Homepage
Seachem market a product called "Stability" which they state is 100% pure bacteria. I have used this, although in a planted aquarium and plants, as Angel pointed out, eliminate the "cycle" problem anyway so I can't say whether or not the Stability did anything. But I am aware of other aquarists who have used it in new tanks and added a few fish the first day and had no losses. In my view, this is exactly the same as "seeding" a new tank with existing bacteria from an established aquarium. Provided there is enough bacteria added to handle the amount of ammonia and nitrite produced by the fish, it will work. I would recommend very few fish to avoid overloading the capability of the bacteria, whether seeding the tank with bacteria or using a bacterial supplement.