roughly 280 Mb download, and yes it has some bugs, and not even all of the melee animations and some other stuff aren't completed yet, but it is nice to see the progress the game is taking IMO.
Besides, when have you ever had a chance to see a game grow to completion from the inside, that is another whole cool factor.
Just if you download it, make sure you put it in it's own folder (default esf_openbeta) definately do not let it overwrite your original version of 1.2.3, this way you can still play that version, since there are some very different things going on in the way the game is played, you can always play either version.
I suggest you enjoy playing both. I do.