If you're a developer then the beta is useful as you'll need to see how you need to change your code or do regression testing. If you just want the latest and greatest you'll be looking at a lot of headaches.
A few of points regarding the beta
- Betas expire, so once you start on them, you have to keep upgrading.
- They're unstable, meaning apps will crash (who knows which one)
- Battery life will be impacted as its a beta and apple is releasing an unfinished OS to developers
- You cannot easily downgrade.
If this is you're only phone or you need the phone for day to day usage, then avoid the beta.
As others stated signing up to for a developer account is not sufficient, you need to be a paid member.