This power-up is found usually when a large battle is following. Thus, they should be used sparingly. They can be found in secrets that are normally hard to access or a suicidal trap.
In Serious Sam: Xbox, if you're playing in higher difficulties, be aware that if you grab it when you have 100 health, your health will start decreasing by 1 HP per second. Thus, strategy is required before grabbing Super Health.
Super Health has appeared in all Serious Sam games. It always gave the player +100 health. They are usually found in Secret areas, such as in Tomb of Ramses III, which holds 2 of them, both Secrets. Otherwise, they mostly trigger traps, such as a lot of Kleer Skeletons rushing at you. There will always be a reason for the game to give you one, or the game just tries to be nice and give you some health.