In the current patch, utilizing DOTs and the Skeletal Mage for pressure, and Vampire Drain + Blastbones + Meteor for burst works out decently well for unorganized combat. The Intensive Mender, Rapid Regeneration, and Renewing Undeath can combine to do a pretty solid job of keeping you alive when the focus fire isn't too heavy, but there's always the problem(s) with "smart" targeting and the Mender bugging out (which, fortunately, doesn't seem to happen nearly as often as it used to).
When it comes to premade-vs-premade fights in BGs, though, I think Magicka Necromancer is probably the worst class for offense. The value of DOTs gets greatly diminished when there are dedicated healers around, and your burst damage isn't nearly as reliable as most other classes. The extra off healing can be nice and all, but I'd still put Mag Necro at the bottom of the list for fighting against organized teams.
'Course, if ZOS doesn't make some pretty substantial changes between the first iteration of the PTS and the version of the patch that goes live, Maigcka Necromancer will soon be utterly worthless for an offensive role in PvP. The single target DOTs are getting their damage cut in half, as is the Skeletal Mage, all without actually addressing any of the issues that Magicka Necromancer suffers from. If you already have all the gear ready to go, don't need to burn any transmutation crystals, etc...I'd say go ahead and give Mag Necromancer a whirl - it's decent in unorganized BGs. Not top notch, but decent enough to have some fun with. But if you're still at the stage where you'd have to spend time and money getting gear and enchants prepared, I'd say hold off and shelve the class for now, until we know for sure what next patch is gonna bring. Nobody wants to invest a lot of time and money into a class that simply won't be viable for anything besides fighting people that are substantially worse at the game than you are.
In the current patch, I'm running Necropotnece, Bright-Throat's Boast, Bloodspawn (both heavy), 3x Protective Jewelry (didn't change from last patch, but maybe should after the nerf) with Spell Damage enchants, and tri-stat drink.
Fire Staff Bar: Degeneration, Invigorating Drain, Stalking Blastbones, Intensive Mender, Consuming Trap. Meteor for Ultimate.
Restoration Staff Bar: Summoner's Armor, Rapid Regeneration, Renewing Undeath, Race Against Time, Skeletal Mage. Life Giver for Ultimate.