They are a nice supplementary material and an engaging read, but are not suitable as the main text for learning a given topic for the first time.
I disagree, I read the Feynman lectures in high school and learned a great deal. His presentation taught me more about how to think about these things then Giancoli did.
Giancoli better prepared me for what the standard format was for test questions, but it didn't really articulate how I was supposed to use the ideas to generate new ones. Feynman's style of connecting claims with whatever you happened to know, is extremely important. Giancoli doesn't demonstrate this style quite as well.
Of course it was my first textbook, so I could go on and on about why I like it...
Actually, this was my experience, as well. But I did not truly learn the subject until I used a more standard text (not Giancoli, but similar). Feynman gives you ideas and intuition, not mastery.
That's not really the point of the lectures. They are meant to be used as a supplementary material.
If you are learning physics fo rthe first time, I would highly recommend Walter Lewin's video course, also available for free:
http://ocw.mit.edu/courses/physics/8-01-physics-i-classical-mechanics-fall-1999/
EDIT: I should say also that an actual textbook with exercises & problems is required for anyone learning physics. There are many to choose from: I used Halliday, Resnick and Krane's. The lab component of a physics course is also essential, but sadly I don't know how to replicate that at home. You get a small portion of the benefit from watching Walter Lewin's lectures, as he does many live physical demonstrations, but that's not the same as doing the experiment yourself (and learning about experimental error and from the mistakes you make).
A related website http://www.feynmanlectures.info/ seems to be maintained by the same people. In addition to the books themselves, one can find an errata, some exercises, etc..
The Feynman lectures on physics are now available to read online for free. This is a classic resource for not just learning physics also but also the process of science and the mindset of a scientific rationalist.