MadWizOz:
Now the question: Are there galleries or series only for members to know? I mean, Is there any chance, that Met-Art release a new gallery (serie) and decide not to show it in the updates page visible to all public?
MET is always announcing all new series by means of showing a 'cover' on the update-site and sometimes is circulating free preview galleries with low-res samples. For members there are from time to time special bonus-series and videos of the known girls. This depends on which membership-level you subscribed.
Similar sites like D omai, J ustTeenSite, F emJoy etc. are doing more or less the same.
From all above sites I think that only MET is worth the money for a subscription.
Of the other sites 'backdoor' facilities are available in abundance all over the internet (especially JTS and FJ) that you would be stealing from your own pocket to signup over there.