They can be useful. About six or seven years ago I subscribed to a few perl ones, then I created a rule in Gmail to tag them all with a label and skip the inbox. Never heard from them again.
Now, though, if I have a perl question, my first step is just searching my email. After years of aggregating, it's got some solid answers.
For me, it was the usenet, and specifically comp.lang.c and comp.arch.embedded. It was quite entertaining and useful, but that was way before it become Google groups and the spammer bots discovering it.