Perks of being a vampire:
>invisibility on command
>can seduce lower level npcs and feed on them (insta killing them)
>vampiric drain becomes good (eventually)
>50% frost resistance, immunity to poison and disease
>massive stat boosts indoor / at night (so far +130 to health / stam / mana)
>a bunch of minor vampire bonuses that scale (attack up, speed up, sneak up, etc.)
Downsides:
>weakness to sun, 75% weakness to fire (goes up to 150% if you don't feed), can't use silver weapons + take extra damage from silver weapons
>Can't use potions (lol)
>Can't use food
>Restoration spells have no effect
>No passive health regen until you're a vampire lord
>all of your stats are reduced during the daytime
>all of your stats reduce and weaknesses increase the longer you don't feed
>dawnguard dudes start hunting you eventually
>reduced effectiveness of almost all wintersun dieties by 75% (this seriously sucks)
>at max hunger all guards in every town instantly attack you
>probably even more shit I haven't figured out yet