In many years of using tbb, its alerted to me to many problems. If I have a line problem 95% of the time I know about it from looking at the tbb graph.
eg. how did I know I had a line fault from 2-4am when I am asleep, I see say 300k CRC errors for the day in the modem, but how do I know the time? the big red splodge on the tbb graph is how.
snadge if the graph shows packet loss then you getting packet loss. but you also have to know how to use this kind of data, and understand that its measuring your connection between tbb and yourself. Having a 2nd graph hosted elsewhere such as
http://fruk.net/index.php?fruk=f8lure can be useful to collaborate results.
Occasionally things can go wrong. eg. occasionally I have seen packetloss show on all tbb graphs for a given isp due to a congested peering link, but when that happens its a case of simply discarding that set of data.
To say tbb graphs are near enough useless is a very strong statement to make which I disagree with.