There have been a fair number of reviews already. Generally if you are familiar with a particular developer or game series, or are highly anticipating a particular game based on various previews, then you may not want to wait longer than necessary to get it.
There can also be price breaks or various bonuses for pre-ordering (though not much with the English version of D2:ED; the German collector's edition was the same price as the regular edition pre-ordering from amazon.de, and one place had a free D2:ED t-shirt for pre-ordering).
