However, I haven't seen anyone, any blog post, etc talk about what type of conversion rate is poor/good/great.
With that being said, has anyone come across any good articles or past experiences on beta landing pages?
The point is that websites don't have conversion rates, which is why there aren't any useful blog posts listing what conversion rate you should be aiming for. Whether your 25-30% e-mail opt-in rate is good depends on what percentage of the people you sent to that page were in your target audience to start with.
If 100% of them were people you spoke to on the phone that sounded super interested in your service, maybe 25% is bad. If they're just people clicking through a link in a guest post you wrote on some blog, 25% giving you their e-mail is probably excellent.
A previous venture, while not measured with the same accuracy, had less than half of the conversion of similar traffic at the same stage.
Usually, the simpler the form, the higher the conversion rate. It's really easy for someone to give them your email but the question is if they will actually become customers. That said, providing an email address itself is a form or validation and gives you the opportunity to reach out to them. Case in point, we received 100s of leads with a simple form but no one converted. Our new venture received just a handful of leads with minimal promotion but we've been in touch with most of them.
You'll also have to consider the quality of the traffic that you're bringing to your landing page.
That said, general conversion rates are useful to some degree. What you'll want to do is evaluate any changes that you make going forward with your current conversion rate to see if anything' changed.
Anything above single-digit % is good, for regular traffic. If the traffic is from people you pitched personally, then of course your percentage won't scale once you get more traffic from people who don't know you.