First of all it’s a good thing you want to try out and help to make Windows 10 better. I did the upgrade on my backup phone and with this post I will try to convince you to do the same. At least not on your primary phone. Since this is a beta release a lot of functions still not work or don’t work as you are used to. Microsoft also states this when enlisting for the insider program.
The version I currently have installed is 10.0.12534.59.
Here are my first experiences using the new Windows 10 for phones pre-release. I will try to update this post when I work more with it or install newer builds along the way. For now I hope it helps you to decide if you are pursuing the upgrade or not.