He's certainly a very popular writer - the Alchemist has sold over 65 million copies, and Coelho is now one of the world's most read authors. However, he isn't regarded as a very good writer by most critics, and overall his books tend not to be much read by people who do a lot of reading generally.
He is easy to read and understand, and his message is basically a simple one about following your dreams, listening to your heart etc. But the quality of his writing isn't high - his style is too banal and repetitive, and the conclusions he comes to are quite shallow. Another problem a lot of readers have with Coelho is that he plagiarises - many writers do this to some extent, but he is often accused of taking material almost word for word from other writers - usually past ones, so copyright laws don't apply - and passing them off as his own. In The Alchemist, for instance, he takes a story from Oscar Wilde about the Narcissus legend and reproduces it in such a way that most readers would think he had actually written it (he mentions the story in his blog, so there is no doubt that this is deliberate). This is just one of many examples.