Barnes tries to give a unified presentation of Aristotle's work, but his method of quoting often from Aristotle's work makes the writing seem choppy. The reader does understand that reading Aristotle's work is even more unpleasant than reading this book. A better example of a short introduction that is done successfully is "Plato" by R.M. Hare, also from Oxford U. Press. I have no quarrel with the quality of the content in the book by Jonathan Barnes, just the organization that makes Aristotle's work seem disjointed and the presentation that makes the book dull.
For someone who is looking for the spark to engage them in Aristotle's thought, or someone who just wants to know a bit about Aristotle, this would be a good choice. There are a couple other really good introductions as well, one is 'Aristotle the Philosopher' by J.L Ackrill, which is a bit more detailed, another is 'Aristotle' by Sir David Ross which is a quite a bit more in depth. Both of these are good as well.