The moment you get this spell, the difficulty of the game will spiral down like nobody's business. It is best used in conjunction with the Robe of Vecna and the Amulet of Power since they will bring down the casting time of Time Stop to 4. Most enemy spellcasters use their first round to cast defensive spells which means you can cast Time Stop first and eliminate them.
A little known fact about Time Stop is that all physical attacks done by the caster during the Time Stop are considered as automatic hits, meaning that the caster will not miss. This is good when you have Melf's Minute Meteors or are a Mind Flayer( from the Shapechange spell).