There is no doubt that the Alexiad is a biased account, but so is practically every other medieval account of this kind.
When this is taken into consideration and the source is looked at in a objective point of view, it becomes clear that the Alexiad gives us an invaluable insight into the reign of Alexius I Comnenus and the First Crusade from a Byzantine perspective. The Alexiad is also fairly readable in comparison to many other sources of the time and it definately benefits from an accurate and fairly modern translation.
This isn't really the kind of book i'd choose to read for leisure purposes. But as a contemporary account for the life of Alexius I Comnenus and Byzantine perspectives of the Latins in the years surrounding the First Crusade, this is possibly the most complete and important account in existence.