I don't think any school but Destruction is useful, to be perfectly honest. Restoration is excellent and - no pun intended - a real lifesaver on higher difficulties, because warding spells and healing will be essential. Alteration is lacking considerably because most of the spell tomes I find increase armour rating and rarely give me useful things like 'open lock' spells, which I'll need because my latest character is a pure Mage archetype. Conjuration is good but it's all about the AI (as in what you've summoned), and if it doesn't act accordingly, which for the most part it doesn't, the school is next to useless.
Other than these I don't know what other Schools there are. I've only just started playing a Mage properly, and it seems Destruction and Restoration will be the ones I use from start to finish. It'd be interesting to see the others at work though.