So I dug out the Empire and Chaos Warriors and let him and his brother go at it.
His brother had chaos warriors on each flank, the chariot in the centre and the general and some mounted marauders just left of centre and the spawn just to the right.
Both generals decreed a genreal advance
The Empire cannon roared, but despite firing four times at the onrushing Chaos warriors failed to kill a single one all game.
The Chaos Lord led the marauders toward the centre. Where the chariot and spawn crashed into the knights. And so a grind began.
The knights quickly cut down the spawn and the chariot didn't kill many of the steel clad Empire warriors.
The verdictWell it took a long time for such a relatively small game. We were constantly looking up rules, but even so, it felt quite slow (and we didn't use any magic).
"Not as good as 40K" quoth the 12 year old - a damning indictment.
The "only four pages of rules" is a flat out lie. It's four pages of rules and then an endless succession of modifiers to those rules, with each unit having at least one (and usually several) special rules that modify in some way the core mechanics. This is probably ideal for 12 year old boys, but just tiresome for me. And yes, I know Warhammer was like that - but I learned Warhammer when I was a 12y/o boy and have spent the intervening 30+ years modifying and updating that knowledge. I would have been no more enthusiastic about learning 8th edition from scratch than this. These days I like simple rules with a small number of core modifiers like Lion Rampant and (to a lesser extent) Hail Caesar.
The battle just ended up as a never-ending slug fest in the middle with only small numbers dieing and no-one running away. I daresay if you could be bothered to master all the rule modifiers there'd be ways to accelerate this but, see above.
So I'm still not feeling the AoS love. I'll play again because Stephen and Gen. B. want to, but it's not really feeling like my thing.