Yeah don't like Earthquake, the fact it knocks down your team sucks, you can get around it by using like Lightnign Strike to position far away right from the start and invis, but still felt meh and damage is less than stellar. Earth has pretty useful summons but to be honest since I have a warrior in my current playthrough, I haven't bothered summoning a pet since like lvl 12 or so, I only barely used the cool witchcraft summon before I stopped, shit's just not worth the AP, the damage it does is pitiful(I mean it does ok damage over 4-5turns, but I end most fights in 3rounds or so).
Earth scales poorly but has some of the best low level spells out of any school, Boulder Bash has a 1turn cooldown once you have enough int so you can aoe knockdown a ton of shit to control stuff early on, Bless is basically necessary on warriors/archers if you want to maximize damage(rogues get the backstab bonus, if you have the right trait), Fortify I haven't used much but it can be useful early game, later 65armor barely makes a difference and Earth Shield is the earliest shield you get which makes it the strongest at absorbing damage since it gets a higher % from int and shields are fucking crazy good to not get raped by large mobs if you're not CCing. But overall Earth is best left at 2, aoe curse and aoe bless are kinda alright but not really that useful(you generally run 2mages when doing 4man groups and 1mage when doing 2man groups so one bless on each mage is enough since spells don't miss and curse is like meh whatever, better use mass slow or mass weakness or just kill shit).
Funny enough, Witchcraft gets the best single target nuke, after having the worst damage spell for most of the game. Vampiric Touch I guess around lvl 10 or 13 whichever it was is alright, does decent damage but the range requirement makes it not so good, however Death Punch is retarded damage, it one shots bosses with proper buffs/debuffs. For aoes, meteor shower(fire) and chain lightning(air) are the best I've found. Ice damage spells are mostly garbage but Mass Slow is a great spell(especially if you have Bully on your physical chars since that's an aoe 50%dmg increased debuff on top of the massive AP reduction) and the stronger heals are pretty nice lategame.
Lategame though, other than Death Punch, no spell really does as much damage as a buffed rogue or warrior. My warrior often whirlwind/dust devil for 3.5k crits(with a 30% crit chance or whatever it's at) and Flurry for like 6k+ depending on crits and shit, it's pretty fucking crazy. Does need to spend the starting turn setting up stuff instead of actually doing anything though but yeah mages do fade out quite a bit dmg wise, still great for control/support and their damage is still decent, just pretty far behind.