Some of the bacteria supplements are good, others are questionable. There is divided opinion among members here (and elsewhere I'm sure) as to their "effectiveness." If you bear with me, I'll explain a bit.
Dr. Timothy Hovanec was (I believe) the first biologist to discover that the second stage nitrification bacteria in freshwater is actually nitrospira, and not (chiefly anyway) nitrobacter. The first of course is niotrosomonas. If you are so inclined, you can read his paper here: http://aem.asm.org/cgi/reprint/62/8/2888.pdf
Along the way, somewhere, he also came up with a method to package nitrifying bacteria so it could be distributed and sold. Originally I believe it was the refrigerated sort, BioSpira may have been the name [don't quote me, going from memory] but later (or whenever) he came up with bottled bacteria. I believe it may have a shelf life (unopened) of 6 months. He marketed it himself until he sold the patent to Tetra, and it is now available as Tetra's SafeStart. This is Dr. Hovanec's formula, and it is live bacteria. You can read about this here: Freshwater Products
Seachem also produce a true live bacteria supplement called Stability. I assume they developed it on their own (they have an extensive scientific lab and biologists). It does work to "seed" a new tank, I have used it and know of many others who have successfully.
What these bacteria supplements do is add live nitrosomonas and nitrospira (and related bacteria) to the water. It is the same, exactly the same, as seeding a new tank with existing filter media, wood, substrate material, etc. that is colonized with these bacteria. It does not "cycle" a tank--and this may be where the confusion and disagreement comes in--it merely "seeds" the tank. One cannot add more fish than the bacteria can handle, no matter how it gets in the tank. The bacteria feed off the ammonia and subsequent nitrite and multiply by binary division sufficient to handle the supply of ammonia/nitrite. Multiplication takes time, about 9 hours for nitrosomonas and 20 hours for nitrospira, under ideal conditions (pH and temperature affect this division).
Of course, live plants as Kym mentioned is another different story, and with sufficient live plants there is no cycle to speak of, again provided the fish are in balance. In non-planted or sparsely planted tanks, I myself would always use one of the afore-mentioned supplements. If the fish introduced are balanced with the bacteria, they will be spared ammonia and nitrite shock and thus be safer and healthier. Just so long as everyone understands the cycle still has to establish itself.
Last word on other "similar" products. If they contain live bacteria, they may be fine. "Cycle" does not, it is chemical, though they say it works the same. I can't comment; but myself I would feel safer using one of the proven bacteria supplements.