It increases the duration of spells by 50%, so I can cast dragonhide for 150 seconds dual cast, ebonyflesh for 297 seconds. It also increases the effectiveness of ebony flesh by +25%, resulting in 375 armor rating with mage armor.
The greater ward gives me +100 armor rating and +100 magic shield. So this means I can achieve 537 armor rating (64% damage resist) with ebony flesh, the lord stone and the greater ward up. You can also dual cast or dual single cast the greater ward, resulting in 657 armor rating dual cast or 637 dual single cast. Which is very close to the armor cap of 667, without any armor.
Any stone you use gets increased by +25%, the lords stone gives me +62 armor rating and +31% magic resist. If you combine the atronach perk and stone, you will be completely immune to magic all the time, 100% magic absorption. Very overpowered though, I use the lord stone.
Any potion you drink increases its effectiveness by 25%, and its duration by 50%, so most self made potions last 45 seconds now instead of 30.
So the archmage robes for example, now give me +125% magicka regen rate, +62 magicka and +18% fortify magic schools.
I can use the conjuration spell raise zombie/reanimate/revelant/dread zombie for 400 seconds when dual cast.
The drawbacks I've found aren't that difficult to deal with, you can keep a companion with you for whenever you need to feed. The -125% (thats changed from -100% as well unfortunately) magicka, stamina and health regen rate in sun I can easily counter with my own enchantments, although I don't need stamina and health regeneration. I usually stay in the dark anyway, you get a power to see in the dark and nearly all dungeons and buildings aren't in the sunlight.
Edit: Just experimented and found out that perks taken whilst being a necromage vampire are boosted by 1.25% too! So the damage perks are increased, all the perks that increase by a specific value (e.g. Increase skill effectiveness by 20%) are boosted.
Te illusion perks that affect how high a level the spells work up to are affected also, some destruction spells damage are increased (not entirely sure how this works, some spells are effected and some aren't), so my flames spell does 27 damage per seconds instead of the 22 it should be (illusion fear perk and +50%).