Apple opened up public beta testing for Mac OS X Yosemite not too long ago, and iPhone users who want to live on the edge can now sign up for Apple's public beta for the latest version of iOS too.
While Apple has done everything it can to ensure iPhone won't be hurt by the latest OS beta, they are called betas for a reason. Signing up for it makes you a guinea pig, willingly accepting potential bugs and issues.
As for what there is to look forward to? Well, if the developer's version is any guide, there's the Apple Watch app, new emojis, support for Google's 2-step verification process, and new languages for Siri.
If you fancy trying the new build then head on over to Apple's Beta website and follow the instructions. You can always delete it and revert back to a previous version if you decide to wait for the final release too.