They would be a good supplement for self learning, but if you really want to learn the topic I'd recommend getting a general textbook and working along with the lectures, doing plenty of practice problems.
I personally think they are great. I was doing an engineering school but I decided to switch to a theoretical physics master for my last year: I wouldnt be doing a PhD now if I didn't have the chance to find these courses on the net by Prof.Susskind.
They cover a lot of material and give you a (strong) theoretical minimum. Of course I had to go a bit deeper during the physics master into some specific materials (statiscal physics or QFT for example), but the basis he gives in classical mechanics (hamiltonian formalism, lagrangian etc) which I never studied before were sufficient to understand the rest. Not to talk about the very good insights he gives to any material, ones you wont find in any textbook..
Listening to 8-12 lectures on a topic without doing any problems will not give you a thorough understanding of it. I remember someone saying how the Susskind lectures are more like a "thin book" on a topic which kind of summarizes it without going into a lot of details. I find that pretty accurate.
For an excellent and much more complete coverage of Quantum and Classical Physics check out Balakrishnan's lectures on youtube (I also have a link to the problem sets if you want them, took me a while to find). For General Relativity and Classical Mechanics, check out Alex Maloney's page where he has audio lectures along with notes and problem sets.
I would say "a thin summary book with lots of good insights". I agree that following Prof.Balakrishnan's lectures helps to understand more things in detail. But as I said, listening to Susskind helps you tackle more complicated subjects much more easily than learning directly from a textbook (which might be cumbersome if you get lost in details).
Yeah, I would say if someone is really serious about learning these subjects, then they may want to start out by listening to Susskind's lectures as kind of a survey of the topic (its not necessary IMO but it can only help provided you have time). After that you'll probably have a good feel of what the subject is all about and will know the basic facts. However you must then follow a textbook and do problems, or listen to the more comprehensive coverage of these topics by professors such as Balakrishnan or Maloney (along with doing the assigned sets of course).
I should also add that Balakrishnan is one of the best lecturers I've heard. Better than any professors I've had so far at college. The guy knows physics inside out and he provides excellent insights about everything he lectures on. I mean how often do you see someone teaching a subject like Quantum Mechanics without referring to any notes whatsoever?