I would say he is my favorite companion. I have never watched Twilight or read the books, btw. What I find attractive is his backstory and how fragile he is behind the mask of a monster and a jester (probably both). He needs help but he does not know how to ask it. At the same time he does have his dark impulses and he has addiction problems (killing you during the bite is just that: his inability to control himself). If you look at his skills, intimidation is not on the list. But he is manipulative and yes, he does want power as a way to get out from his position. I think in his world, there are only predators and prey. He wants to be a predator. It is not surprising, considering what he endured throughout the centuries.
The way he is written I think he is open to influences, perhaps even an alignment change. I also think his romance is also the trickiest of them all. It can end horrible and I am not sure a true happy ending is possible (the best you can hope is a bittersweet one). I also think that if there is some sympathy between Tav and him (approval rating), he is one of the more reliable characters from the current party. At least until you meet Cazador and he faces his dilemma.
I also think his opening scene is great. Why? If this person had the chance to slit my throat but he did not do it, I might not fully trust him but I will still view him with more trust than somebody who did not have this chance. At least in the current environment.
@GM4Him
I think you do overestimate Astarion's ability to plan. He is crafty but he does not strike me cool headed enough to weave sophisticated intrigues. I think Gale and Shadowheart might fall more easily into this category (though this will be a gross simplification). I think Astarion just messed up and was caught. But he is good enough to... improvise.
