I've had iOS 7 beta 1 on my old iPhone 4 for about two weeks now, ever since the WWDC keynote wrapped up on June 10th and the dev portal came back online with the download link. I am not putting this on my daily phone. Doesn't matter how badly I wanted Control Center the second Craig Federighi swiped it open on stage. My 5 stays on iOS 6 until this thing loses the word "beta" from the corner of the lock screen.
If you're thinking about trying this yourself, a few notes from someone who already made most of the mistakes.
You need a paid developer account. That's $99 a year. The free membership just lets you build and run your own code on your own device through Xcode for a week at a stretch, it doesn't get you into the beta program at all. I already had the paid account from a contract job last winter, so this cost me nothing extra, but if you don't already have one, think hard about whether an OS preview a few months early is worth $99 on a phone you're going to have to babysit constantly.
The download itself was a mess that first night. The dev center got hammered right after the keynote ended, and the restore image (just under a gig) kept dying around the 60 percent mark. I want to say it took me four tries and almost until midnight before it actually finished. Not really Apple's fault, everyone with a developer account was hitting the same servers at once, but still, plan to do this on a night you don't need the phone for anything.
What's actually good: Control Center is legitimately great, and I use it more than I expected to. Swipe up from the bottom, flashlight and brightness and airplane mode are right there, no more digging through Settings. AirDrop for iOS also works better than I thought it would, though I've only had one other person nearby with the beta to test it against, so take that with a grain of salt.
What's rough: the visual redesign is a lot to adjust to, and I say that as someone who wanted skeuomorphism gone. Some of the new icons look like they were handed to five different interns and each one got a different box of Pantone chips. The Weather icon in particular I still can't parse at a glance. And the system font, that ultra-thin Helvetica Neue weight they're using everywhere now, is genuinely hard to read in direct sunlight on the 4's screen. Maybe it looks better on a 5. I don't know yet because I'm not risking my 5.
Also, small thing nobody warned me about: the new parallax effect on the home screen, where the icons shift depending on how you tilt the phone, made me a little seasick the first day. Turned it off in Accessibility settings within about twenty minutes and haven't missed it.
Third-party apps are where this beta really shows its age. Twitter's app still renders with the old-style icons and a UI that clashes badly with the new system chrome. Instagram hasn't been updated for the new look either, so it just sits there looking out of place, like a guest who didn't get the memo about the dress code. None of this is a knock on those apps, they can't update for a beta most of their users don't even have yet. But it does mean the "this is what your phone will look like" experience is currently maybe 40 percent there.
Battery life is bad, worse than I expected even accounting for beta software. I'm getting through maybe half a day of light use before I'm hunting for a charger, and this is a phone I don't even carry around, it just sits on my desk while I poke at it between other things. If you do install this, don't put it on the phone you actually need to survive a workday.
One more thing, unrelated to any of this but it's been stuck in my head since the keynote: Phil Schiller stood on that stage and said "can't innovate anymore, my ass" right before they showed off the new cylindrical Mac Pro, and I have thought about that line at least once a day since. Doesn't matter that the machine won't ship for months. It's just a good line, and I respect a company willing to be a little petty on stage in front of five thousand people.
Anyway. Back up your device first, use an old one if you've got a drawer with a spare in it like I do, and don't say I didn't warn you about the battery.