Sometimes they'll win you the game, sometimes they'll lose it for you. En masse they can be deadly, but it seems to depend too much on what you opponent builds infantry wise.
Totally disagree on never giving them to Tommies, you need the sight range of pyrotechnic tommies to use them effectively as their biggest strength is their range. Can be quite effective with an AEC willing to dive too. If treadbreaker worked then I think Piats might come into their own though frankly they are a bit of a curate's egg.
I think they have a couple of useful niches, actually. They are excellent building killers, and while they are not generally useful damage dealers against tanks or light armor they do force your opponent to keep moving (i.e. forcing down his accuracy.)
They're definitely not something to build a comp around, but I think they have a real role.