It is easy to spend a full day hopping on and off the Bus Turistic.  The drivers and staff are very helpful and the commentary easy to use and understand.  The route options cover the full city and regular buses ensure a seat when reboarding.  The tour is great.

Base price is 21 Euros for 1 day and 27 Euros for 2 days covering all routes (as of August 2009).  Some hotels can offer the bus tour at a discounted rate.  Check with the receptionist.  There are three routes to choose from that goes all over the city.  You are going to need at least one full day, maybe even two to see everything.  Included in the fee are a booklet with discounts for various attractions and restaurants and the map of the city. 

For more info and prices check out: http://www.barcelonaturisme.com/Barce...