If I'm not mistaken, you get 100% resistance to Disease, Poison, and the higher stage vampirism (Level 4) gives you unique abilities (i.e., embrace of shadows, which lets you go invisible), but at the final stage of vampirism it is also considered a crime and people will attack on sight, forcing you to constantly hide in the shadows. Also, unlike Oblivion, you no longer burn up in the sun; rather, you lose the ability to auto regenerate Magika, health, and Stamina during the day, so you're almost forced into the night if you don't want to rely on potions. So if you're okay with most of that, then you're good to go.