This weekend Blizzard is running a fully open beta for Diablo III. Multiple of my Facebook friends were really excited for the announcement and one of my LoL teammates had bad lag last night as his roommates were busy downloading the client. The game itself launches in 24 days and I'd have to think the beta will be pretty polished by this point. This is likely either just a server stress test, a PR move, or an attempt to get a lot of people to download the client now to lessen the strain on the servers come launch.
I for one am really uninterested in playing in the beta. Characters are capped at level 13. I believe you're still capped at the first act. Nothing you do this weekend will carry over to your live account. The way I look at it, I have lots of games I want to play. I have no need to play an unfinished game with no save state. Games also tend to have a number of hours before they get boring and I don't want to waste any of my D3 hours now. Maybe if I had nothing else to do, if I felt I'd gain really useful information, or if I wasn't able to afford the game when it launched I might feel differently but as it currently stands not so much.
Don't get me wrong, sometimes beta tests are really good. I was involved in the Magic Online alpha/beta and it was great. No features were turned off and I actually couldn't afford to play Magic in real life as much as I did in the test. No cards carried over but I did learn a lot about the cards. (Probably my best standard season ever was after that test. Go Tog!) Perhaps most importantly I was actually able to impact changes to the game. I also spent a lot of time in the StarCraft II beta. No really chance of making changes there but at least the game was fully functional. They were doing the test as a balance test so they needed to get people playing against each other with all the units as they tweaked numbers. Each game is self contained as well so you wouldn't lose any progress (except towards achievements I guess).
At any rate, if you're interested in playing a little D3 this weekend you can go play in the beta. I won't see you there!