It worked very well for me.

My ISP in Germany was lending cables from another ISP. They failed to fulfill the "up to 100 MBit/s" claim all the time.

So I set up a Raspberry Pi (which is a bit flawed, as the network interface on the older models was too slow, to begin with) and let it run speedtests every 5 minutes for four weeks.

The graph looked terribly enough that I was allowed to change ISPs without any cancellation period or something else.

Sometimes they care, probably because I blogged and tweeted about it, too.