The GOOD:
To begin with, as soon as you become a vampire, you gain a 100% percent resistance to Disease and Paralysis. In addition, you'll gain bonuses to a host of different skills and attributes. Each of these gets a bonus of five points per day that you go without feeding, up to a maximum of four days. So the first day you're a vampire, you'll gain +5 to all of the following attributes and skills. On the second day, they'll all be +10, and so on, up to the fourth day, when they all gain +20.
Here's a list of the affected skills and attributes.
- Strength
- Willpower
- Speed
- Acrobatics
- Athletics
- Destruction
- Hand-To-Hand
- Illusion
- Mysticism
- Sneak
- Protection from Normal Weapons
Suffice to say, whether you're a warrior, a wizard, or a thief, you're going to find that these bonuses are general enough to be useful to your class. By the fourth day without feeding, your character will be significantly more powerful than he or she was before.
The Bad:
With that said, though, there are plenty of drawbacks to being a vampire. As soon as you become a vampire, you'll obtain a small but noticeable weakness to fire damage, which grows more severe as you proceed without feeding. By the fourth day, this vulnerability maxes out at 50%.
On the first day of your vampirism, you and your pale skin won't necessarily feel any ill effects from sunlight, apart from making some itchy flaking. On the second day, though, one point of damage per second of exposure will be meted out. This increases to four damage a second on the third day, and eight damage a second on the fourth. (For the game's purposes, "day" is considered to be any time between six AM and eight PM.) This constant damage prevents you from waiting while outdoors or using fast travel. (If you attempt to wait while outdoors so that you wind up waiting through some sunlight hours, don't be surprised if you end your wait period and instantly die.) Overcast skies or rain will reduce this damage somewhat, but you'll still usually find yourself making a mad dash for an indoor area to wait out the sunlight. (Luckily, your enhanced speed will make this easier for you.)
On the fourth day of vampirism, your appearance will have changed enough to make your affliction clear to the rest of the world. While you won't be attacked outright or assaulted by guards, most of the characters in the game will simply refuse to speak to you, or at best tell you to get the hell away from them. You can overcome this once per day with your Vampire's Seduction power, or perhaps with other charm spells if you happen to have some Illusion abilities.