I right now have ntop, darkstat, vnstat2 and bandwidthd installed. None of them are truly usable in my setup.
darkstat runs only on one interface, even though the web configurator allows the selection of multiple interfaces (UI bug)
I have to monitor more than one interface => useless. But at least, it recognizes the IPv6 tunnelbroker interface properly, and handles its IPv6 traffic seemingly correctly. Also, there seems to be a bug, where after a system upgrade, one has to go to the settings page and hit the save button, before it starts up again.
bandwidthd, unlike the rest for some reason to be found in the services menu, can seemingly also only run on a single interface, at least the UI allows only a single interface to be selected. From all I can do it can't handle IPv6 => useless.
ntop, doesn't recognize the tunnelbroker IPv6 interface, and lists it as BSD loopback interface with an address of 0.0.0.0
vnstat2 is full of old php that throws errors, can only work on a single interface, and doesn't seem to be doing anything beyond what the RRD graphs do, so I'm not even sure why this package (still?) exists.
RRD graphs etc. work fine, but they don't allow for any finer-grained reporting.