The main things that the reviews prepared me for are
(a) the resolution is still pretty low
(b) the head tracking is still fairly sloshy
which are very true, and detract a little from the awesomeness but I think if these get fixed in the consumer version (which I hear they are) shit is going to get very very real.
Creepers blowing up in my face and grass blocks that really *feel* 1m x 1m x 1m and the water level going over my head and sprawling bucolic vistas oh man.