McKay and others don't claim that they have found proof that life existed on Mars 3.6+ billion years ago, but rather that their findings are consistent with the hypothesis that life once existed on Mars. Two types of criticism can be brought to bear:
1. The features that they isolated in the rock as suggestive of past presence of micro-organisms on Mars are due instead to contamination of the sample either during the flight to Earth or during testing;
2. Each single clue, such as the presence of PAH or of iron sulfite and magnetite, can also be explained as due to a non-biological cause.
The best evidence of the presence of Life on Mars would be to find little green men/women (Where does it say that beings come with only two genders? Why not three or more? There are actually organisms, on Earth, which have more than two genders). Short of that, or of finding living organisms, the question of whether life existed on Mars a few billion years ago will be settled if it can be ascertained that certain features in rocks could have been left only by living organisms. Here is more about the debate whether or not McKay's results could be ascribed to past presence of life on Mars.
1. Skepticims raised about discovery of life on Mars(from CNN Interactive)
3. Views of Mars from the Lunar and Planetary Institute