Don't Install If You Use Your Device for Work or School
If you need your phone/tablet and its apps and services for school and work, you probably don't want to install the iOS 9.2.1 beta on your device.
Beta updates can cause degredation to app performance. You might see an increase in the number of crashes, you might see something else. It's difficult to predict.
This uneven performance is precisely why Apple no longer allows beta users to post reviews on the App Store. Too many people were complaining about problems related to the beta which, again, is early software that's not meant for the average user.
If you rely on you apps during the work or school day, or if you have a big project coming up, be smart and take a rain check.