I have a NL635 with the "Fast Insider" option, so I can't offer complete advice. However, I had it on my daily driver for a few weeks. It is very unstable, and it takes getting used to the common freezes and crashes, etc. I also notices that many apps do not work, and a few of the stock apps are broken, like lock screen settings. I did welcome the new features, however, and if the phone you plan on installing the Beta on is not your main device, I would suggest that you do install it. For me, it got too crash-prone, and I had to revert to my Technical Preview for WP 8.1. For the few weeks I had Windows 10 for Phones, I got used to the additions, but since come of the apps that I need were not ready and compatible for the Beta, I was forced to downgrade.
I sincerely hope this helps, and if you have any other questions pertaining to the aforementioned issue and anything WP related, feel free to ask!