What is it?
A bloody, pulpy orb that sends any creature it hits into a blind wrath. As soon as the pheromones - or whatever's in that gross-looking blob - make contact, the target will turn against the nearest living being and begin attacking it mercilessly.
When should I use it?
First, let's emphasize when NOT to use Enrage. If you're battling a single Splicer, hitting him with this plasmid will only cause him to assault you with more ravenous ferocity than before. Bad idea. Accidentally unleashing Enrage in the heat of a fight is easier than you'd think, too, so choose its location in your lineup carefully when at the Gene Bank. For example, don't stick it between Incinerate and Electro Bolt. Plasmid cycling can get rather feverish in moments of desperation...
In groups, however, Enrage can be a life saver. Mobs of Splicers don't appear often in BioShock, but when they do, it's overwhelming. Splatter one with this plasmid and - bam! - you have one fewer foe and one brand new ally. Better yet, peg them all with the red orb and watch as a blood-soaked, curse-laden cage fight ensues. Just remember to retag the Splicers occasionally if you want the freak show to continue.
A final note: Enrage only affects organic targets, so don't bother with cameras or turrets.
Does it work against a Big Daddy?
With luck and patience. Regular enemies rarely beat a Big Daddy one-on-one, but Enraging a Splicer can still distract him for quite awhile, giving you loads of preparation time. A crowd of Splicers, on the other hand, can whittle away a good chunk of the Big Daddy's health before he dispatches them. Just resist the urge to join in the fun, as the Big Daddy might decide to ignore them and turn on you.
And no, you can't Enrage a Big Daddy against another Big Daddy. Except for special exceptions in the storyline, it's impossible to locate two Big Daddies in the same area. Sorry!