Does Zend pdo_pgsql Adapter allows to use most benefits of PostgreSQL? Or there is some restrictions?
It depends on what you are doing. Usually PDO support almost all supported features - and some features (like prepared statements) are used simply than in original (native) interface. What I can see, a PDO is used well in applications. One important feature, that is not supported (what I know), is COPY API - special interface for bulk load.