Seconded. I spent about two weeks studying with the mobile version of this site in my free time and managed to pass the technician, general, and amateur extra exams in one sitting.
Thirded, I used hamstudy to pass all of the tests and after a few years on the air I finally know enough to understand the ARRL manuals. I'm reading the extra manual and it's pretty incredible, they walk you through the theory of building a transceiver starting at doping silicone to make semiconductors.
Fourthed, I always recommend hamstudy.org to people. My wife and I both used it to study for a bit each night and easily passed our tests.
We live in an area that doesn't have good cell coverage, or some days the tower just goes down and there isn't any service where there was great service the day before. Even cheap, basic radios are very useful.