This is probably not the best edition of On Liberty - I would recommend Stefan Collini's Cambridge University Press edition for scholarly apparatus and background information. Generally the text is accurate (the various errors from the early editions have been corrected by John Gray). A few new, if unimportant, misprints have crept in, however. The footnotes are next to useless, giving explanations such as a description of Rousseau as "a philosopher". For those who simply want the text, however, this edition is perfectly fine, and has the benefit of the two lesser-known essays Considerations on Representative Government and The Subjection of Women in addition to On Liberty and Utilitarianism.
On Liberty has been a hugely influential, and also misinterpreted, work. Contrary to expectation, Mill formulates a fairly authoritarian model of government designed to root out behaviour which causes harm or offence to others, but the novelty in his treatment is his defence of the liberty to do in private what you want (so long as you harm no one), however immoral your behaviour. It may be a cliche to say it, but these issues could not be more relevant today, in debates about drugs and pornography especially.
Rather than the drier philosophical style of some of his works, these popular essays are vigorously written, in a clear and energetic style. There could not be a greater political polemicist than Mill - and this should be seen in the light of his life, where he actually put into practice what he preached in these essays - equality, liberty, and toleration.
Now, of how many philosophers can that be said?