Taxis are an excellent method of getting around Porto.   They are inexpensive and easy to find.   Additionally, they can be hired for day tours around the area by visitors who wish to get some guided information about the major tourist attractions of the area.   Taxi service is particularly recommended for getting from hotels to the wineries which are popular tourist attractions in the area.   For an example, see http://www.tripadvisor.com/ShowUserRe... .
The taxi drivers are said to be friendly, English-speaking and helpful, although of course this will vary from cab to cab.   Two of the most popular cab companies in Porto are Invicta Radio Taxis do Porto and Radio Taxis Os Unidos.   More information on taxi companies in the area is online at http://www.infotaxi.org/city-884.htm .   

Rental cars are not recommended in Porto.  They are relatively inexpensive; however driving is generally considered more of a hassle than a help in this area.  The streets are crowded with heavy traffic and bad drivers who are easily confused by the random assortment of one way streets which make up Porto.  Travel by taxi is highly preferred.  Public transportation is also considered to be better than hiring a rental car.  For more information on public transportation, visit http://www.tripadvisor.com/Travel-g18... .