I've never done one and I just had my 88th book accepted.
So yeah... not sure how useful they are in general.
Classes are only as good as the person teaching them, and it's been my observation that creative writing and similar classes, whether they are taught online, in person, by a well-known author (commercially successful), or by someone only a handful of people have ever heard of, tend to be biased toward that person's style of writing, the genre in which they write, whether they loathe a certain type of writing (genre fiction, for example), and a host of other subjective slants that affect what the members of the class get out of it, or don't get out of it.
I mean, let's face it. Unless you're teaching grammar and punctuation basics (which these types of classes assume you already know), any evaluation of your assignments is going to be subjective because there are no standards for what makes a "good" essay, a "good" poem, a "good" personal experience story, etc., etc., etc. Why, even right here on our very own site we've had people who have taken such classes feel they have made it as a gifted writer because of those classes, and who also felt they had a right to judge others' work because of having participated in them. I don't think so, and neither did a lot of people on here.
Personally, I don't think creative writing or any other writing classes can "make" you a good writer, if you don't already have the gift inside you. Think of it in terms of a musician. They aren't taught. They're born. They have the gift inside them. They can take classes to hone their skills and learn the basics of their craft (music theory, composition, etc.) but when they sit down at a keyboard, or pluck a guitar, or begin to write down notes, the gift they were born with shines through. Writers are the same way, IMHO. Yes, we need to learn our craft - spelling, grammar, punctuation - and it doesn't hurt to learn the mechanics of a story. But when a born writer bangs out a story - even without honing all those skills - the work has a spark to it, even when it's in a raw form. You can tell the person has a true gift. Once they hone their skills, the writing only gets better.
When I was in college, I dated a man who was a born musician. He would sit down at a piano and pluck out the parts of a song he'd written, hearing the way each instrument should sound together in his head. It was fascinating to watch. It's the same with me when I have a story I'm working on. The characters talk to me. I can write the story in my head, all day long, no matter what else is going on. I didn't learn that in a class. I could
never learn that in a class because you can't
teach that to someone. It's always been there, inside of me. All classes did were teach me the fundamentals and the basics. And to be perfectly honest, I learn more from edits and from reading others' work than I ever learned in high school or college English classes. Just like a musician has to practice, practice, practice to get even better, writers need to
write to get better at it. But without the basic skill set there to begin with, nothing is going to happen beyond a certain point.
I don't have a lot of musical ability. Some, but not much. No matter how much I practiced, or how many books I read on music, I would only get so good because that's not a gift I was born with. And to be honest about something else, I've read some books and stories by people who believe that because they did an online course or took a few creative writing classes, they are a gifted writer. Not always so. Only my opinion, of course, but I wasn't moved at all. I think they wasted their money to be perfectly blunt.
So yeah. By all means, if you feel you might get something out of a class, take it. But don't go into it expecting to be made into a best selling writer if you're struggling with the process, or struggling to get things published, or aren't getting any positive feedback from professionals - agents or publishers - about work you're submitting. The classes probably aren't going to help, beyond giving you a chance to write. And you can do
that without spending money.
OTOH, if it's being given by an author whose work you admire, or by someone you would love to get critique from, then it might be useful for you to take the class. But be judicious and approach any of them with caution. A lot of them promise things they can't possibly deliver.