Im on PC and it just came on sale for ￡7, is it worth it? It didnt get very good reviews and apparently there are alot of bugs in it.

Yes, it's most definitely worth ￡7.

It is worth it... Becareful of some bugs though...
I once bought GTA IV for $5, and I had trouble with it...
But it was worth the $5 in the end...

Not a big fan but the crossbows make it worth it.

The "Summon Durnehviir quest" (playin on pc) does not work for me, if I had know this I never had bought Dawnguard. I have started 2 new games but the quest did not work in the first game and not in the second game

Yes, Dawnguard is worth the $7. You get a lot of content for that price. Dawnguard is a step in the right direction imo. The quests are more enjoyable than the vanilla quests. You get dragon bone weapons, you get a new mount, you get a new beast form with its own perk tree.

Certainly.

Of course. It was a great DLC. I felt it was a little over priced when it came out for $20 but if it is on sale you should definitely get it.

Dawnguard is a massive buff to the vanilla game. Very much worth it.

Yes definitely worth it and no more bugs than any other part of the game, maybe less actually.