Having the useful quality of stacking with the innate magic-resistance of Bretons, this perk is something that everyone wants, but not everyone can reasonably get. If you’re not investing in Transmutation, after all, you will already have to pick up two other perks before reaching this one, and not everyone has that kind of spare room after the main things they want to do with their perks. If you can afford it though, you should get it no matter what kind of character you’re putting together- 10-30% resistance to all spell effects is nothing to sneeze at (unless someone’s trying to kill you with a Frost spell).
Essentially a superior version of Alteration Dual Casting that will, if needed, stack with the aforementioned Perk, Stability goes a long way towards making your ‘-flesh’ and Paralysis spells more usable, without requiring that you put away your weapon or attack spell before casting. A far more worthwhile perk than Alteration Dual Casting, you should always wait to take Stability and see how you do with only that boosting your spell durations before going to the trouble of taking Alteration Dual Casting. You may well find you no longer care about Dual Casting when you can use your Alteration spells at 150% duration anyways.
A Perk for any and all mages who find themselves running low on Magicka later in the game for whatever reason, the Atronach effect makes fighting enemy casters a breeze and probably means you want to start killing them after the melee and archery enemies, instead of needing to go after them first. 30% of the Magicka cost of the spell hitting you can mean some pretty big surges of Magicka, and goes quite well with Magic Resistance to let you tank a lot of incoming spell damage and shrug it off- probably by using your Restoration talents to heal yourself with the stolen Magicka.