While I don't have hands-on experience, from what I understand from users is that this device is for fairly advanced play, i.e. your sub should be comfortable taking something that large in the ass, when it is expanded.
And when you release and remove the device, there is the potential of the claws pinching tissue between the claws as they close up, so be quite careful about this.
Also a shopping hint, when this device first came out there were some sites selling it for a ridiculously high price and others at a more reasonable price.