I jumped in to iOS 14 with public beta 3 two weeks ago. My experience has been really excellent for an early beta. I've had almost no issues. Even battery is decent, like 7.5 to 8.5 hours on my iPhone 11 Pro which normally gets 9-10 hours. The Alto games are a bit wiggy but all the apps I rely on are working perfectly, including banking apps which have not worked on early betas of previous major new iOS versions.
Of course, YMMV.
In reading this thread, I see lots of people having issues with beta 4. I don't know if that means it's a step backward from beta 3, since people reporting problems are generally not a representative sample of those who installed the beta - people who aren't having problems are less likely to visit a forum to say so. What is probably a more reliable indicator is Youtuber Zollotech's community poll which he runs for every new iOS beta. The poll asks if the new beta has been "great", "terrible", or "okay with some bugs". That poll usually gets thousands of responses within the first few days it's up. If you divide the "terrible" number by ["great" + "okay with some bugs"], you get a rough estimate of the probability you'll be in the "terrible" group if you install the beta. He doesn't post the poll immediately after the new beta drops, since it takes a few days to get a sense of how well it's working. So if you want to use the poll results to decide whether to install the beta, you have to wait until probably Sunday-ish.