I play as a fighter but have been impressed with the damage done by mages. Did an escort mission for a Mage in a dungeon and was impressed at how hard she could hit the enemies.

I like the summoning stuff and resurrecting opponents to fight for me but heard there were problems with destruction spells.

What are the viable Mage builds and which ones are the most fun?

conjuration and/or illusion are probably the most powerful

Agree. Destruction alone isn't all that great, but if you combine it with conjuration and match the elements (e.g. chain lightning and storm thrall), you have a pretty deadly combination.

This.

With destruction as third-most important.

And be an Altmer for the +50 magicka.

Edit: As a robe wearer, the Lord Stone is great as for damage mitigation.

I have no problem with Destruction magic...take the perks so your spells will be stronger. There are staves and scrolls and all kinds of weapons and tools you can use. You said you were a fighter so I'd suggest Destruction magic and I find Alteration has some amazing spells. I'm planning on making a pacifist character and will use Illusion and Alteration for that...it'll be a challenge. My favorite Destruction spell in my current build is dual-casting an augmented Firebolt spell. It makes a great sound when you fire it off too!

Also, I've found the Altmer Highborn power is a lifesaver in hard battles.

So the other question is robes v armor. I know that batltemages split the skills but I'm not entirely clear down which line. Given the perks, I'd probably see more new things going pure Mage but am not clear on all the trade offs. Hate the usual robes the necromancers wear. My Mage should have pants.

Get illusion and conjuration they are probably the two most powerful. Then consider destruction to supplement each of those two spell schools. Then think about maybe enchanting to reduce the cosy of higher level spells. Consider alchemy for buff potions and restorative potions. You might want to get restoration as well for healing and the circle spells.l

If you're building a mage because you were impressed with how much damage NPC mages dealt, you will be disappointed. Their version of Destruction annihilates the version that you will be using.

Illusion and Conjuration are godly when they get maxed though.

Sneak mages are cool I.e ShadowMage. You can go anywhere from illusion and conjuration to resto and destruction. I use a little of everything and have quite a lot of fun

Well my mage uses all schools of magic as well as alchemy and enchanting. I will most likely get smithing up to Dragon armor as this version is using light armor. If you want pants you can use enchanted clothing, it takes a bit to get it to the level that it can compete with robes.
Destruction has its place and uses but you can't just cast damage spell all the time. There is actually a in game book that hints at this through a story about a missing princess that was suppose to have been assaisnated.

Seriously, destruction is still a great skill till you're like level 40+, with the 'impact' perk its quite overpowered.

That is a lot of food for thought

My pure mage uses light armor, but not a lot of points in it. I also use enchanting - not power-leveled - and enchant the light armor with mana regen, additional mana, health, whatever a mage can use. I do see a good supply of light armor so you can increase your armor level by just picking up stuff from dead enemies/npcs. Then go back and enchant the new pieces and level enchanting that way. It's all nice and slow.

I do have a 2H greatsword - enchanted - as last defense in case an enemy gets close. Doesn't happen often. Dual wield/cast chainlightning FTW. Buy the book from the mage in the Jarls office - Whiterun area. Get soul tap soon to power your gems.

Good luck.

PS: Also get the perk that "staggers" the enemy. Nobody gets close to you.

Conjuration, it's awesome.

Conjuration is arguably the best single school, as it has so much variety. Back it up with either Destruction or Illusion or even both.

summoner is really fun, conjure an atronach and bound sword and go to town on whatever you find...and really thats all you need to beat just about everything (well after you can summon dremora lords)

Destruction is really good imo before level 35ish then it kinda quits scaling and you just impact stuff until it dies...it can take a little while lol

Ok I have tried playing a few mages. My current one got it right. Currently level 54, playing on expert, and has no problems. Also playing dead is dead.

Destruction: If you plan on playing on Master then the damage will be a problem. Any other setting and the real problem is not the damage, it is the cost. Spamming ranged attacks that do 200+ damage + 100 magic damage + insta kill once they drop the target's health below 15% + auto stuns anything you hit with it. Now throw in two dremora lords as body guards, the ability to turn invisible at will, etc. and you have one really nasty character. The problem is spamming the above spell will use a 1000 magic in no time at all. Want to play with destruction past level 50 on expert.... the higher level perks + 0 cost destruction spells(or even 90% cost reduction) is the way to go. Oh and the mentioned spell was Thunderbolt dual cast with impact perk, both shock augmentation perks, and disintegrate perk(insta kills at 15% health)

Current character strategy. Ok I knew going in that destruction spells cost to much and I would use enchantments to lower the cost. Stone= atronach. Wearing robes. First chance I found some greater and grand soul gems I started enchanting gear with fortify destruction. Didn't get my cost to 0 early on, but I did get a 40% cost reduction by enchanting 4 items(easy even with a lower enchant skill), then 50%, 60%, 75%, etc.... 100%. So as the game progressed and I got more powerful spells they were also getting cheaper. This also solved the other big problem with mages. Most people who complain about mages find themselves always running out of magic so they keep putting all their level ups in magicka pool. Then they find themselves at level 40 with 150 health... and a deathlord one hits them with an Ebony bow(happened to my first mage). With cheaper destruction spells you suddenly don't need as much magic since you rarely cast the other spells over and over in a single fight like you do destruction spells. Not needing as much magic= more level ups go to health. More health means surviving is a whole lot easier.

Other schools. Illusion can be just plain game breaking. I rarely use it. Conjuration is fun, and twin dremora lords can be really powerful but powerful enemies will still kill them.

Destruction spells to use and loose.
Early on, firebolt is good. Once you have fireball you will live by it, it is the best all purpose kill spell until you start to run into enemies that need to be killed with the expert spells. All ranged expert spells are awesome. Master spells all svck because of the time to cast vs what they do.

Wall and rune spells are a waste. Don't really do enough damage to be worth it. The wall spells sound powerful, 50dmg per second, but it only hurts the target while they are touching it. A fast enemy will run through it in less than a second... so you can gather the damage there. Now the wall spells are very pretty, and tons of fun against mudcrabs, wolves, and horkers. But if you use it on a bear you are going to get mauled. I like them just for the visuals. Note if anyone who is not a companion walks through them... plan on a fight.

Fire is best early game. Shock once you get the disintegrate perk. Frost is ok for slowing enemies down, but all dwemer constructs, all undead, all nords(most common race), most animals, half of dragons, and I'm sure something else are either resistant or immune to frost while only a few enemies are resistant to fire and shock so it is not a good all purpose spell.

This always makes me cringe. I've seen a lot of actual complex builds (http://go-dl.eve-files.com/media/0802/skillchart.png) in games.

And here we have people discussing what of the six linear perk trees to use. You get enough perks to max out EVERYTHING magic related and there are no mutually exclusive choices. HOLYCRAP, IT'S SO HARD!