This graph is a good general indicator.
As you can see, Necrons are in practice solidly average, and all armies are more balanced than perhaps word-on-the-street might make you believe. Deamonhunters perform very badly and Orkz iz made fer fightin' an' winnin'. But the most important information is that the spread across all armies is only 22% or so, if my back-of-the-envelope maths is right. Which is pretty damned close.
that said, this graph is from February '09, two months before the new Guard Codex.
(
http://confoundingblog.wordpress.com/2009/02/26/is-warhammer-balanced/)