No QoS, uPnP/NAT-PMP never worked, assigned IPs never stuck (making manual port forwarding useless), and fell apart under moderate load (five, six devices connected to the network, maybe three of them actually using *some* data).
However, once Tomato was loaded onto the router, we noticed a marked improvement in performance, and all the features listed above magically started working (until I was a little overly aggressive with QoS and accidentally choked off access to the Internet via any device, but that's a PEBKAC problem).