There's a few great vampire mods out there and I intend on letting my character become one for the first time since I've started playing Oblivion. I know there's some difficulty in being a vampire and many NPCs won't really be very polite towards me, I'm assuming. If I do become a vampire, can I still complete most (if not all of the quests), including the main quest? I have never completed the main quest and I'd like to, but I don't want to become a vampire and regret it later on (even with a previous saved game).

If you feed often enough you can complete all quest as Vampire. Basically on the lowest level of Vampirism (24h since feeding) nothing bad can occure, ppl do not notice you are vampire and you do not take sun damage. But you get least ammount of bonuses. On the other hand higher levels you get gradualy lowered disposition from many NPCs, that result in them not speaking to you, but none of them will attack you on sight, also you take higher and higher sun damage. But all can be solved by feeding and hiding.

for more info:

http://www.uesp.net/wiki/Oblivion:Vampirism

I would suggest not to read point "2.1.1: Becoming Vampire by Choice", "2.5: Curing Vapirism" and "3: Vampire Nightmares" if you do not want spoilers.

Good news. Thanks for the reply, Willyish.