I have not used that service before, but Criterion is English-oriented and dedicated to only the best quality- hence their high, sometimes unfathomable prices. It's a "try-before-you-buy" thing so I would imagine that the streams are DVD-quality, and if you like one of them enough you get that much off your purchase.
Of the featured films available to watch online, I highly recommend Antonioni's L'avventura, Ramsay's Ratcatcher, Bergman's The Virgin Spring.
But I especially recommend Sluizer's The Vanishing (Spoorloos). The damn thing is creepy as hell, and very underrated.
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