Yep. I'm running iOS 8.3 beta 2 and it's a lot less buggy than 8.1. That trend is rumored to continue with iOS 9, so I'll definitely be installing.
I think it's going to have separate betas like they did with Yosemite last year. Developer betas and public betas. In that case I'll still test the developer betas instead of the public beta. It gets less updates than the developer betas.
If they do it this way, I think I would try the public beta. I would expect it to be more stable.
Would I be able to update to the official firmware without any hassle if I was running a beta?
Generally.
I would only volunteer if the update process was pain free.
If your expectations are that any part of the process be "pain-free", then you should definitely not be testing beta software. Beta testing requires a willingness to live with potentially serious bugs and to deal with restores, reinstall, resets, etc.
I would only volunteer if the update process was pain free.