Reports I've read on the ASUS units is that they're prone to overheating. But with millions sold, a few reports might have more weight than warranted.

I always orient my networking gear for optimum airflow, which usually means vertical alignment on a wall somewhere.

My Netgear WNR3500L gizmo runs lukewarm to the touch -- room temperature, about 18C. It's also one of the few routers I've encountered that is designed for vertical orientation.

Cheers