Yes it's a very viable playstyle, especially if you get your Enchanting up since you can put a Fortify Block and Fortify Health enchantment on your shield which helps a lot in close quarters combat. Also, you're lucky in that wielding a shield does not count as armor for the Mage Armor alteration perk, so you'll still get double boosts from Oakflesh Stoneflesh etc even while holding the shield.
The only true problem is that you can't really afford to invest in Stamina (or much Health for that matter) near the beginning of a game while using any form of mage, so Shield Bashes are limited. However, once your Enchanting gets up and you can put multiple Fortify Destruction etc enchants on your clothing, the extra Magicka means little. A Shield Mage can very happily have something in the region of 400 Magicka, 200 Health, 200 Stamina (before enchants) by level 51.
Hope that helps.
ps.
Spoiler The Gauldur Amulet from the "Forbidden Legend" quest is excellent for this type of character, might want to keep your eyes open for that
