This is the best physics book I have ever studied, and absolutely indispensable to anyone who has to solve real-world problems in electrodynamics!
Jackson's treatment of physics is elegant and crystal-clear throughout, and he generously includes material on Legendre functions, spherical harmonics, etc. that undergraduates may not have been exposed to. However, when it comes to the mathematical verifications needed to read the book with understanding, he doesn't hold your hand. This is as it should be! Physics is not the same as mathematics, but the latter is an essential tool that every physicist must master.
If you go through the book conscientiously checking and deriving all the equations, you'll end up filling a large notebook (amazingly, I didn't find any errors -- this book is rock-solid reliable). But the great thing is, you'll never have to do this again! Then you can reread Jackson without getting snowed under by what are essentially routine, but sometimes tedious, mathematical verifications; and you'll be free to savor the physics and really appreciate how great the book is. If you want some worked problems, look at The Classical Theory of Fields in the Landau-Lifshitz series. At the end of each chapter in Jackson there are excellent problems that will test your understanding.
The negative reviews of Jackson can only indicate a problem with how physics is taught in graduate school. Jackson's book has to be read carefully at least twice, and preferably N times; perhaps this is too much to expect from harassed graduate students in physics. I'm a mathematician by training and was able to study Jackson's book on my own and enjoy it without being rushed for time or having to worry about being tested. With the possible exception of the volume on quantum mechanics by Landau and Lifshitz, which again requires a lot of mathematical verifications, Jackson's book has no rivals.
It's a pity he didn't write any other physics textbooks.