Yes, I'm a 30x500-er, and yes, I'd recommend it. "3x5er?" Not sure. We don't have a good collective noun :-)
I'd say the course is absolutely worth it if you're serious. As you've seen, it's not cheap. If you're only mostly serious, the price is a kick in the pants. You may actually do the work to avoid wasting multiple thousands of dollars. I don't think I individually needed that -- nobody seems to think it's for them! But empirically and numerically, that seems to help people. Isn't it nice to be able to hack the sunk cost fallacy to work in your favor? Relatedly, yes, I think the class will absolutely help other developers create products above a certain level of seriousness.
So here's the hard part, which is both the best and worst part of the course. As developers, you and I both want to write code and have people buy the results. Doing that is, like, maybe 30% of building an actual product, if you're lucky. 30x500 isn't about writing code. You already know how to write code and you don't need to pay to learn. You mention that you're already working on "the guts of a web app", which is a good sign, I was doing the same thing... But don't get too attached to it.
30x500 is a combination self-help and marketing course. That's what I actually needed. You will be ignoring that half-built web app and instead finding an audience (potential customers), figuring out what actual problems they have, and building something to help with that. Which is how I wound up shipping Rebuilding Rails instead of a freemium build-a-portfolio site for developers, which I had mostly built already. The class takes awhile. It has to, because it has to pry your darlings from that death grip you have on them. You'll see most of them strangled by the end of the class, which is exactly what needs to happen.
You need to refocus. Not just your eventual marketing efforts, but your from-the-start product design and development efforts. You have to solve a problem that somebody will pay to solve. If your half-a-web-app is like my various half-a-web-apps, that's not what you did. I certainly didn't survey the market first and build them because somebody was lining up with cash to solve a problem.
I built them because they seemed like a good compromise between "fun to build", "within my abilities" and "somebody might want it, maybe", which I judged by whether it was like some startup I'd heard of.
That compromise is death.
Somebody said that making money is so hard you can't do it by accident. You have to actually focus on it. But you can't not-focus on it for awhile and then start focusing on it. You have try to do something that people actually want (and will pay for), right from the beginning.
30x500 is a long series of calling you on your bullshit, one lesson at a time. There's some how-to as well, but those are the parts that you could find by Googling how-to articles. Don't take the class for the how-to articles. If you do you'll be disappointed. Take the class for the series of articles about being brave and focused, and here's what being brave and focused looks like during this one specific activity.
It emphasizes the process, and the process is simple, sometimes almost insultingly so. That's because you're going to, at some level, be terrified of what she's advising. It requires a lot of arrogance to make something from nothing and then tell people they should buy it because it's awesome. If you can follow a simple set of directions, consistently, week after week, 30x500 will be worth it to you.
Do that despite being terrified and disbelieving, failing repeatedly, getting little or no customer feedback early on, and generally having every reason to quit. You'll get there.
I'm not the most successful 3x5er. That might be Jarrod Drysdale or Brennan Dunn, I'm not sure. But I'm pretty sure I'm in the top handful. I'm also very good at following a simple procedure, consistently, while terrified and disbelieving.
I don't think that's a coincidence.