Schengen-not-implemented is a member of the Schengen Agreement but has not yet fully implemented it. For EU and EFTA (Iceland, Liechtenstein, Norway) citizens, together with those of Switzerland, an officially approved ID card (or a passport) is sufficient for entry. In no case will they need a visa for a stay of any length. Others will generally need a passport for entry.
Travel to/from any other country (Schengen or not) from/to Schengen-not-implemented will (as of now) result in the normal immigration checks, but travelling to/from another EU country you will not have to pass customs. However, if Schengen-not-implemented normally requires a visa for your nationality, this may be waived if you already have a valid Schengen visa.
Inquire at your travel agent or call the local consulate or embassy of Schengen-not-implemented.
The visa list is already consistent with those of the Schengen countries fully implementing the agreement.
Only the nationals of the following non-EEA countries do not need a visa for entry into the Schengen Area: Albania*, Andorra, Antigua and Barbuda, Argentina, Australia, Bahamas, Barbados, Bosnia and Herzegovina*, Brazil, Brunei, Canada, Chile, Costa Rica, El Salvador, Guatemala, Honduras, Israel, Japan, Macedonia*, Malaysia, Mauritius, Mexico, Monaco, Montenegro*, New Zealand, Nicaragua, Panama, Paraguay, Saint Kitts and Nevis, San Marino, Serbia*/**, Seychelles, Singapore, South Korea, Switzerland, Taiwan*** (Republic of China), United States, Uruguay, Vatican City, Venezuela, additionally persons holding British National (Overseas), Hong Kong SAR or Macau SAR passports.
These non-EU/EFTA visa-free visitors may not stay more than 90 days in a 180 day period in the Schengen Area as a whole and, in general, may not work during their stay (although some Schengen countries do allow certain nationalities to work - see below). The counter begins once you enter any country in the Schengen Area and is not reset by leaving a specific Schengen country for another Schengen country, or vice-versa. However, Australian and New Zealand citizens may be able to stay for more than 90 days if they only visit particular Schengen countries—see the New Zealand Government's explanation.
However, all British Overseas Territories citizens except those solely connected to the Cyprus Sovereign Base Areas are eligible for British citizenship and thereafter unlimited access to the Schengen Area.
Further note that
(*) nationals of Albania, Bosnia and Herzegovina, Macedonia, Montenegro and Serbia need a biometric passport to enjoy visa-free travel,
(**) Serbian nationals with passports issued by the Serbian Coordination Directorate (residents of Kosovo with Serbian passports) do need a visa and
(***) Taiwan nationals need their ID number to be stipulated in their passport to enjoy visa-free travel.