If you buy a ticket for a 1hourtrip on the river (10€) at the sao bentotouristoffice you get a free tasting of 3 port wines at quevedo caves I highly recommend it!!!
Without stating the obvious, the port is good. Taster options are a good shout, the staff are super friendly. Not too expensive, and if you take the cable car at the moment you get a free taster.
Portwine is much much better at Taylor's. Grab free tastings here, but then go to Taylor's to feel the difference between "okay" port and the truly excellent one.