Never tried it but you do have to make sure you don't get medicated feed as I hear they counteract each other. As for if the vaccine works, probably depends on the area. I've had one issue with cocci, but that was correlated very well with a sudden change in weather. Heavy rain after being brooded off the dirt so no immunity built up.
As with all meds. I am sure it works, but there will be always exceptions. Like the flu vaccine, people get it get the flu from the vaccine, and can easily get a strain of the flu that was not in the vaccine.