1) The reviewer's first duty is to his readers, not to the artist, therefore he must be free to say a CD is rubbish when that's his honest opinion. Reviews which could give only favourable verdicts would be just one more form of advertising, and useless to the readers they're supposed to serve.
2) Many reviews do contain a good deal of factual information, and that's often useful. There's no simple right or wrong answer to the question of whether a CD is worth buying or not, however, merely a range of opinions which will differ sharply from one individual to the next. There's no such thing as objectivity when it comes to this aspect of reviews, and nor should there be.
3) The critic's job is to articulate his own opinion in an interesting and informative way. The fact that he hears such a lot of music in his chosen field should allow him to build up an expertise and offer insights a casual listener would miss. The fact that he gets so much practice at this helps him communicate his views with greater clarity and precision than the average fan could muster.
4) If you read the reviews pages in any music magazine regularly, you will soon get a sense of which critics you trust and which you don't. With luck, you'll discover someone whose taste coincides with your own, and feel confident in regularly following their recommendations. Sometimes, it's equally useful to find someone you know you'll always disagree with, and do the opposite of whatever he suggests.
5) In an age where there's simply too much of everything, we need useful filters more than ever. A well-written review will give you a far more useful basis for deciding whether something's worth buying or not than any 30-second clip on Amazon or a hopelessly biased plug from the artist's girlfriend.