Betas work differently for different companies.

My last experience with Beta testing in WoW was that once I got the invite and could log in, it was pretty much set for the duration of the beta.

There are a few things folks need to be aware of:

1) Beta is beta. It is not a finished product. There are going to be hiccups and glitches. You help the beta a lot by simply reporting the errors. That's what beta is there for - to report that your Goblin's eyebrow is floating 6" above his head.

2) You may or may not get access to certain things. For instance, right now, level 85s can't go to Pandaria. Also, things may change every build. A zone you can get to may no longer be available. A talent you like may be gone. Your Goblin's eyebrows may disappear.

3) You cannot keep anything you get, you cannot preserve any toon. Once the beta servers are shut down, everything you have done in beta is gone. Everything you do in beta will have to be done all over again in live. You'd be surprised by how many people keep asking if they get to keep their beta toons.

A lot of folks go into beta thinking they're simply getting "early access" to the finished product, and that's not what beta is at all. The best you can take away from the beta is experience.

Now, if you want to contribute feedback as far as game play and class mechanics at a time when Blizzard is still refining, then getting in early is a good thing. If you just want to test drive the game when it's mostly done, but not yet golden master, then you'll probably be happier towards the end of beta when Blizzard is stress-testing.