No they are not. It will give you some notion of the popularity of a site but time and time again people looking at this have found that the data is directionally and discretely wrong.
There are quite a few services like Alexa, including Compete, Nielsen Online, Comscore, Hitwise, Quantcast, SimilarWeb and various google tools – all are useful in some way but not to take at face value, traffic wise due to various reasons.
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The reason a site like Alexa is not accurate is because they rely on the usage of toolbars that collect traffic data, but the audience that use them is notoriously unrepresentative of the population at large.
The only way to get really accurate data is to get access to web analytics numbers somehow. In the absence of this, Nielsen Online and Comscore data is useful, but expensive (and they certainly also have their own limitations).