I was bored with my lvl 41 leader of the dark brotherhood(and unrelenting one shot-one kill machine) so I gave a mage a try.
I'm of course not speaking of those silly armored mage, why would a mage have armor, he has the powers of the UNIVERSE AT HIS FEET!(yes I'm a megalomaniac mage :p).
And damn, that first dragon kicked my ass, I had to do it 3 times before I finally finished him off(in normal difficulty) >.<.
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I've done almost the exact same thing recently, having started with a now 40ish DB/Thieves Guild/bow sneaky character. Mages feel like they could use some loving; either you wear heavy armor and have to train yet another skill, or you wear none and have to rely on your low starting magicka and the occasional staff drop, if you're lucky. I'm glad I am specializing heavily in Alchemy.
On a side note, Destruction talents give your staff more kick depending on the element being used. Also, the more advanced your Destruction skill, the less charge it takes when used. Nice touch, at least. But I don't like not regenerating magicka when using my shitty streaming ice or fire staff.
I'm doing the same thing, but on the other side. Yesterday I finally reached 100 with my destruction skill, so I joined the Legion and changed all my oufit, from a robe and no weapon to a one-handed weapon and light armor. Fun ensued, as now I'm not able to finish the first Legion's quest.
It's like starting the game again, but with more powerful foes compared to a new game.