One small annoyance is that it needs to be plugged straight in to your M300 via a USB cable.  The Modular Vault will sell for $1,500 on its own.

More exciting is the prospect of the Integrated Disc Vault, which has a player built right in so you don't have to worry about hooking anything else up.  It's also capable of holding 300 discs, three times that of the Modular.  It's going to be coming out sometime in 2011, with June as an expected date.  The price isn't set yet, but Kaleidescape says it'll be "no more than $6,000."