Do other vampires (normally hostile ones) still attack you when you yourself are one of them?

Yes.

Is it worth becoming a vampire or it svcks so badly and waste of time?

It's useful for stealth based characters, but the constant feeding may become a hassle unless you're into heavy RPing. There's nothing really special about vanilla vampirism in-game. The Dawnguard vampires get all the cool [censored] it seems.

Thanks. Is feeding difficult? Is it hard to find sleeping people?

Thanks for answering my question..seems less attractive now to be a vampire over joining the werewolves.

I doubt that find sleeping npc's will be any harder then it was in oblivion.

Naw, feeding is easy and nearly everyone cashes out around midnight in any given town. The real pain about vampirism is that the longer you go without feeding the more powerful you are. The draw back is that once you reach stage 4, every single NPC will attack on sight. And trying to find a sleeping NPC to feed on just so you can exist in society can be very difficult.

Even with level 100 sneak and every perk taken in that tree, town guards will still be very alert to your presence, even if they can't see you. Copious use of invisibilty and muffle will be needed quite often so you can just get by. :/