I have used this book a lot while studying English and I think IT IS WORTH BUYING for everybody who regularly uses English language and wants to speak and write correctly.
Practical English Usage contains hundreds of entries on English grammar, from elementary to advanced topics and some entries on other issues (pronunciation, usage of problematic words, style etc.). The entries are well selected with and well indexed, so I found answers for the vast majority of questions I have ever tried to look up.
Keep in mind, however, that the book can be used primarily as a REFERENCE. It is NOT structured for systematic, regular study and it is too dry for that purpose.
If you want to learn English grammar, I recommend that you get this book, together with Basic English Grammar by Eastwood and Mackin (with the exercises). Use Eastwood and Mackin's book for systematic study and complement it with Swan's book whenever you have any specific question. These books together cover ALL you'll ever need on English grammar (except if you need to be a professional in the field - such as a translator, university professor etc.).