Yes, the Neptune Lounge is reserved for the top two levels of suites - the Penthouse Verandah Suites and the Deluxe Verandah Suites. The "perks" of the various "levels" of suites are pretty well laid out on the "Suite Life" section of the Holland America website (under Accommodations)
Now, of course, I guess things could vary from ship to ship in terms of how closely the "rules" are followed... but when we've cruised HAL in a Deluxe Verandah Suite, the concierges (who supervise the Neptune Lounge) pretty much know each of their passengers by name, and know who's in the various levels of Suite.
If it's not too far out of your price range, I'd strongly advise upgrading from superior suite to the deluxe level (Penthouse is "over the top" price-wise). The payback interms of perks is well worth it!!!