Basically you just get all the system updates a month early. When new features are added to the Xbox One or they fix an issue with the OS you get it a month earlier. Getting DLNA support a month early was big for me. One of the benefits right now is early access to OTA support. The potential downside is that things might not be as stable as the standard OS builds. I have been in the preview program since the first month without any real issues though.
When there is a closed alpha/beta they usually give all the preview members access too. I took advantage of that for both Halo 5 and Evolve and there are others I wasn't interested in. Being in the preview program certainly isn't a life changing experience but there are a few perks and not much, if any downside.
