I was a member years ago, quit, and rejoined this year.
It's worth it, depending on what you expect to get out of it. For instance, when I first joined, I thought I could then put "member of HWA" in query letters, but they will warn you against doing so.
There's a good forum where you can discuss horror and promote work (to a degree). You get free access to put things in the newsletter's classified section, and announcement section, which is worthwhile. You become more visible for possible Stoker Recs (though of course being a member isn't mandatory), and you can recommend work for the Stokers.
There are other advantages. Networking advantages. Access to certain information you might not otherwise have.
But again, in the end it's more about what you expect to get out of it as compared to what you *do* get out of it that really matters. I'd suggest taking a look at their member's page, perusing the benefits, and deciding if it's something you want to be a part of.
Edit: It's also worth mentioning that this is one of those "you get out of it what you put into it" scenarios. There are plenty of opportunities to become more active within the group. They are always looking for volunteers for different committees and whatnot. There are also local chapters of the HWA you can become active in, or if one isn't close to your location, you could probably start a local chapter.