I've done some cursory tinkering, if I notice any great wins/fails or can devise a test scheme, I'll try it out and report back. There's now a LuCi module for QoS on OpenWRT (not sure how long it's been around). There weren't any answers from actual DD-WRT users, but the prevalent opinion seems to be that QoS in DD-WRT (when configured via the GUI) is broken.