This Spell affects the very fabric of time itself; slowing everything around you to a crawl while leaving the user or caster immune. Higher levels allow the caster to maintain this state for longer, and bring the world nearly to a standstill.
Slow Time is very useful against large enemies like trolls. You can get in many hits with little fear of retaliation.
Summoned creatures do not slow down with the rest of the world. If a creature is under your control it will fight at your speed.
Slow Time can be a hindrance when you are fighting weaker enemies as a flourish will knock them down, causing you to have to wait until they (slowly) get back to their feet.
In Fable, Slow Time can be used to cheat at the timed tavern games (excluding Card Pairs in Bowerstone, where spell use is disabled). Casting the spell and then initiating the game will cause the effect to last for the duration of the game, allowing for easy wins. In Fable: The Lost Chapters (Xbox), this effect was removed, but in Fable Anniversary it has been restored.
Oddly, Slow Time does not actually seem to slow down the passage of time, as in the time of day. This becomes apparent when the spell is used next to the statue in Lookout Point, as the statue continues to tick at the same rate.