If you'd ask me when I thought 5.7 was stable enough to use? That would be version 5.7.4. It took some time to get there and the first few versions had some bugs. It came with a whole lot of new features and code updates, which made it possible to use composer and have namespaces (yey!).
Now, version 8 is upcoming. You think, why change version numbers from 5.6.x to 5.7.x and then 8 (or 8.0.x)? Well, Portland Labs (the company behind concrete5), rethought their versioning. They even made a versioning guide for that. I think it's better to do versioning this way, as a dot (.) release, mostly means "slight" changes and the first number is "critical" to identify on which version you are. Version 5.6 and 5.7 are 2 complete different things, so didn't really do any good if you ask me. But hey, they got that fixed.
January this year, 2016, Franz Maruna (CEO) announced this new version and some of its features. In this annoucement, you will hear a lot about Data Types/Express Objects. This is a huge time saver in verion 8. But graphic-wise, there is a huge difference too: Fieldless Forms. This basically means input fields have no border, which make forms look clean. In turn, it isn't obvious to many what to do, where to click. We can debate about whether it's good or bad, if it's gonna be the future or not. Only time will tell. If you do want to read about the how and why, you can read that in the "Fieldless Forms" blog post on concrete5.org.
Then in June 2016, the snapshot version (alpha) got released to test out functionalties, let them know about bugs, make Add-Ons compatible if they aren't yet (coming from 5.7). There have been a few alpha versions since (up until Alpha 5 if I'm not mistaken). Currently we are at Beta 1, so we are getting close to a release. Don't take my word for it though, there are still some issues on GitHub at the time of writing. Just a little over 400, so it could take another month (or two) before this will see the light.
As with version 5.7, I'd recommend to wait deploying version 8 on live sites until at least 2 months after the first release. Always be sure to have a stable enough version on your production site. I can't wait for it to be released though, as Express will be friggin' awesome. Mark my words.