I have used a lot of bacteria in a bottle in my time. I have found some better than others as well as cheaper as others. I always seem to have some on hand and mostly use it to help the cycle process but I always cycle for long periods of time.
I can say that in the past due to my lack of planning I have had to set up on the fly a hospital tank and would have to mix fresh saltwater place a sponge that I have had in another system and dump a lot of bacteria in the system and add the fish.
I have saved fish that way so I would say that it does help but I have not made it my go to when I need to set up a tank on the fly. The best bacteria bang for your buck IMO Was Doctors Fosters & Smith now sold at Live Aquaria. But once the Drs foster smith website got shut down the bacteria price seemed to double for some reason so I am not sure where I will be getting it from now on.