Some people here complain about the writing style of Heywood, which I don't see. As a textbook it may be boring, so if your professor made you go through it cover to cover, that's her or his fault and that way this book will kill you!
During my BA and MA I used it more as a first introductory step to research or writing essays. That way you don't forget anything. So an essay on, e.g., federalism, I'd read the chapter on decentralisation (and only that) and then take it from there.
So I use it more as a work of reference. I'll be honest. Long after my Master's degree I still often go back to this book. Not only to get the basics right, but to get back to basics. Because -- let's be honest -- political science isn't exactly rocket science and the field hasn't developed many irrefutable key insights that it takes superior intelligence to grasp.
If you could split political science into on the one hand political philosophy and classical scholarship, and on the other hand the behaviourist science-y stuff, I feel this book is more of the former and gives the latter it's deserved (read 'small') space.
So, as a work of reference, wonderful.