You might be tempted to install the iOS 10 beta on your iPhone or iPad, but trust me when I tell you it's not worth the risks or headaches yet. We're not even to the public beta yet, and by then I would expect things to be in better shape, but this is not a release that Apple wants in the hands of everyday people.
Before I get into the issues with this early release software, I will say that of all the new betas from Apple, the iPhone one seems the most solid. You can already take advantage of some of the cool things Apple showed off in the keynote, including all the new iMessage stuff, new Apple Music app, widgets (for Apple apps at least), and smart Photos search and albums. These features all work quite well, but even still there are some drawbacks that keep me from recommending this beta to anyone at this point.
NOTE: I will continue to update this post as new betas are released, as eventually there will be a beta that I will feel is safe(wish) for most people to install.
Apps weren't built for iOS 10 and there are definitely some quirks
I have yet to run into any apps that are flat out broken, but there are a few apps that are a little less enjoyable to use than they were on iOS 9. And to be clear, none of this is to slight app developers. iOS 10 is a developer beta and is not intended for actual use as your daily phone. The fact their apps work at all is more a stroke of luck than anything else.
YouTube works, but I have had issues with it recognizing rotation so videos all play in portrait orientation, even though they should go landscape. There's also a glitch with buttons flat out disappearing (although still functioning) along the top as you navigate the app.
Reeder mostly works, but I have had many crashes when trying to open articles, save them for later, or sharing them via the share sheet. Again, not unusable but frustrating.
Apple's own App Store app is having a hell of a time installing apps for me. It will install maybe 1/3 of the updates it sees I have to download and flat out refuses to download the rest. A restart got a few more to go through, but some apps are just stubbornly not updating.
Safari gets unresponsive somewhat regularly, as the entire page will just freeze in place for 5-10 seconds while it does...something. It comes back and acts like nothing happened, but we both know it did something weird.
Facebook has had some crashing issues for me. It's not every time, but I've had it when trying to simply post to my timeline.
That's all I've run into myself, but you can find a wonderfully complete list of what apps are working well and which are having issues over at MacRumors.
System bugs make this a less than solid experience
In addition to apps having compatibility issues, iOS 10 itself just has some bugs that are a little jarring.
Battery life is far worse on iOS 10 than it is on the stable version of iOS 9. I have an iPhone 6s Plus and routinely make it through the day without needing to charge my phone, but iOS 10 is chewing through my battery and I am topping my phone up every few hours on the beta. I feel sorry for those with smaller iPhones that don't have this enormous battery.
The system also suffers from a general sluggishness, as it's clear this is not an operating system that has been optimized for speed yet. It's not ripplingly slow by any means, and sometimes it is actually quite fast, but I can feel the difference in the overall performance.
Rotation is completely borked. There are massive issues with the accelerometer simply isn't convincing the phone it needs to rotate when turned sideways. This means some of my photos are not rotated properly, videos in all apps play in portrait when they should be landscape, and images in my Twitter timeline are all small as I can't rotate to see them in their native orientation. Restarting the phone seems to fix this, but it starts up again after a few hours.
Today view widgets are broken, seriously broken. Apple's apps all work pretty well, but all my third party apps with widgets are a hot mess right now. They can't resize themselves to the right height, so they're all the same height whether their content fits or not. And those that look funny in their new widgets are the lucky ones because many apps' widgets just flat out don't work. Television Time, for example, has a really nice iOS 9 widget, but it doesn't load in iOS 10.
And then there's all the little things. Animations going in and out of apps is funky, bringing up the multitasking screen can grind your whole system to a halt, and Spotlight search is painfully slow. There's more than that too, and you never quite know when you're going to run into something else that's just...off.