Without QOS: Average latency spikes into the 60-70ms range while downloading large files.
With QOS: Even when downloading large files average latency only touches 30ms. An increase of only 5ms from my minimum.
This is with a TP-Link WDR4900 running a recent snapshot of OpenWrt Barrier Breaker and using the default qos-scripts package with my speeds set to approximately 90% of actual speed (determined by speed test).
I get no complaints from other users of the connection. Even when multi-gigabyte files are being downloaded or torrents are being shared the connection remains responsive for people on VoIP calls, Skype, or gaming.