To being a vampire, there are ups aswell as downs. Amon the ups comes the ability of certain skills and attributes being raised - the more "vampirical" you are, the higher they're boosted. Another use comes with a number of Powers (one Lesser and three Greater). When you're a "grade 1" vampire (recently fed) you can use a Lesser Power that gives you Detect Life 100 ft and Night-eye for 30 seconds. As a grade two vampire, along with certain stats/attributes being boosted by 10pts, you gain a Greater Power (can only be used once every 24 hours) that gives a Charm effect on the person you cast it on. As a grade 3 vampire, along with earlier advantages, the same stats are boosted even further, and you gain yet another Greater Power, which can demoralize weaker enemies and casts a silence spell. As a grade four vampire (as strong as you can get), the same stats are boosted even further, and you gain a power that gives complete invisibility for 300secs, and Night-eye.
Of course, there are downsides too - mainly Sun Damage and the inability to speak to NPC's. As a grade 1 vampire, there is no actual downside, besides a slight weakness to fire, except that you'll have to feed every once in a while, if you don't wish to "increase vampirism". Grade 2 vampires and higher, suffer from "Sun damage" (being outdoors between 6:00 am and 8:00pm will slowly drain your health until you get inside a cave/house/ruin) and further increasing your weakness to fire. At grade 4 vampirism, you will only survive in the sun a few seconds before you die, you're incredibly weak against fire, and almost every NPC (everyone besides guards, I think) will scream at you to stay away from them.
There, up's and downs of being a vampire. And, from personal opinion, I can tell you that it's definitely worth it.