iOS 8 beta 3...
...Now in regard to stability, good battery and keeping your iPhone from getting warm, I'll just state the following:
1. The latest iOS version (non beta) should be quite stable on your device. It was on mine.
2. Battery life usually depends on one's usage. The more you use it, the more power it consumes. Some people have services running in the background and are unaware of it for the most part. Twitter, Facebook, location reminders, games that have access to location services and notifications, etcetera can all help drain your battery. I recommend you go through the settings and make changes accordingly.
3. If you have a smartphone and you use it, it is going to get warm, especially if you use it continuously for a certain amount of time. That's just how it is.