It depends on you and on the listserve.
There are usually several types of people on a listserve: posters and lurkers. Posters are the ones that keep a listserve alive. They ask questions. They share information of interest to the group. They answer other people's questions. They encourage others. They share ideas. Lurkers are the people who are reading, but not participating in the conversation. They don't comment, nor start new topics, nor share good and bad news. Does this mean they can't get anything out of the posts? Of course not. They can glean lots of information from what others are saying. But...if they have a question and don't ask it on the listserve, how will they get it answered?
One of my friends had been lurking on a listserve and because I "out"ed that she was there (she had invited me to it), she decided she'd better introduce herself. Nervously, she wrote a post of intro and commented on a topic that the group had been discussing. She asked me to look over her post before she sent it. "Is it okay?" she asked. "Definitely," I told her. "Go ahead and post." She did, and guess who commented?! Andy Boyles of Highlights. Just by making an intelligent comment on a listserve she had a short conversation with an editor.
By chatting with others, I've also made friends on listserves. This Saturday I get to meet one friend face-to-face for the first time. Is that cool or what?
Listserves come in a variety of kinds: regional, topic or genre, general writing, organizational. What's the right group for me, may not be the right one for you. I like trying out a listserve. It's like going to a club meeting. If you enjoy the people you meet and the topics of conversation, you'll come back. If not, you won't. If your focus changes, you may need a new listserve and may let an old one go.
They can become timewasters if you are involved either in ones that are very busy with many many conversations, or if you're involved in too many listserves. I like getting my listserves in digest format versus individual emails. I can scan the topic headers and skip any that aren't of interest to me. It also helps me limit the time spent.
So how do you find listserves? Most of the ones I participate in were by invitation or through a writing organization. But you can also find them by searching yahoo or google groups. Here are some I found that way:
childrensbookandarticlecritiquing - the title says it all
Childrens-FandSF-Writers - the F stands for fantasy and obviously SF is Science Fiction
childrens-writers - a discussion group
childrenswriterstoday - a forum for writers, poets, illustrators, editors and publishers of all genres in the juvenile to teen market to announce their latest news, reviews, columns, books and publication works
fantasyweavers - an online critique group for writers of middle-grade and young-adult fantasy and science fiction
internetchildrensstories - this is a club devoted to writers of children's stories and their readers
Northwest Independent Writers Association - for writers of any kind
When searching make sure you check the statistics (latest activity; members; and if it is important to you, whether the group is moderated or not). Some groups will be open and others closed. Some groups may want to know something about you before adding you; others have no vetting process.
If you've never tried one, ask other writers or illustrators what listserves they like. Then join one or two. Lurking at first is okay, but remember you'll get more out of it, if you post, too.
image courtesy of morguefile.com