In the fiction, it's always the build up to the Necrons being unleashed that makes for the interesting reading. Once they're out and just walking around bwooping things (or however their undead gauss thingies sound), the army doesn't have much character, at least compared to the rest of the 40k universe. The anticipation is in the build up to the invasion. By 40k standards, the Necrons are pretty tame when it comes to how they make war, I think. They don't go in for as much planet rape as Chaos or Tyranids do. By the time they do their stuff, everyone who would be shocked by it has already been bwooped.
The Deceiver and Nightbringer are there to give them the oooh ahhh factor as has been said. Whereas Orks or Chaos Marines, one with an attitude usually makes for a decent character by 40k standards. You have to go pretty high up the food chain with Necrons to get to something with personality enough to make repeated encounters worth reading.
The Necron theme is pretty similar to this and while the faceless oppressors are really really freaking horrifying in the beginning, even they have to take a backseat to something bigger and ultimately more interesting. You can't just be under siege by a faceless terror machine for eternity...you go numb and it stops being scurry, especially when it has, until you get to the top of the food chain, zombie-like motivations and little else.