Sylar certainly takes at least some of his victim's brains, and incorporates them into his own. The exact details of this are never explained. Sylar's original, individual, personal power, is "fixing things".
I thought it was very poetic, in the "5 years in the future" episode, that many of the original goodies have turned into baddies, and many of the origanl baddies have turned into goodies. It reminds me that in real life, humans are so very complex in their motivations. Most people think they are doing the right thing, when other people think that they are doing the wrong thing (Just look at politics and business). It's not as simple as good people vs evil people.
The plot was an obvious **** Germany parallel, and so I got to think again about how people behaved in those times, in order to survive, and all the horrible choices it imposed on people. Often the only way to stay completely morally pure was to surrender to being killed.
It's also an examination of a slide into conditions like **** Germany, and how you can get really far into that comdition without even noticing. I personally think we've started taking away far too many human rights, in the name of "security". My government now practises daily torture on people held for years without trail, and still thinks it's an enlightened, free democracy.
Back to Heros. This show has never dissapointed me so far. The cast has been uniformly excellent, and as strong as each other. It's a very rare show in that something of importance to the plot happens in every episode. You have to pay attention.
The only other show on commercial tv that has been as important to me as this one, is the X-Files.