hi folks, here is my take:
the beast is a really slow sheet..as slow as it gets..the main difference between beast and nightmare is, that nightmare has a lower trajectory making it hard for the opponent to topspin the blocks and its substantially faster. the advantage of the nightmare, due to speed, is that its easier to attack with it. beast has the advantage of substantially more control and blocks that become really short, the thicker the sponge the slower, however the trajectory goes slightly higher. the beast is also more forgiving concerning fast no spin loops and angle changes that go along with spin/no spin.
concerning spin reversal: (actually spin is never reversed, its passed on): supposedly the nightmare should be used with slower blades, whereas the beast needs something faster. i never tried the nightmare on a slower bat. the spin reversal on fast blades is very similar, there might be more "reversal" with slow blades and the nightmare- but as mentioned, no hands on experience with that.
ione thing to note when some people state that both have no substantial spin reversal whilst others claim the spin reversal is great: the spin reversal depends on two main factors: the blade you are using and your opponent. if your opponent is a rather weak player or doesnt loop with much spin, it shouldnt surprise that the blocks cannot create any reversal out of thin air. thus the contradiction in reviews is a result of different blades, possibly wrong technique and surely the opponents the reviwers have been playing against.
its a great rubber, which i can only recommend, but like all slick antis it takes a lot of practice, this isnt plug and play!
