Gavin Menzies has his own section in the FAQ. His work has some serious methodological flaws-- you may be interested in this review by Robert Findlay that is referenced in some of his answers. To briefly quote:

Menzies flouts the basic rules of both historical study and elementary logic. He misrepresents the scholarship of others, and he frequently fails to cite those from whom he borrows. He misconstrues Chinese imperial policy, especially as seen in the expeditions of Zheng He, and his extensive discussion of Western cartography reads like a parody of scholarship. His allegations regarding Nicolo di Conti (c. 1385-1469), the only figure in 1421 who likes the Ming voyages with European events, are the stuff of historical fiction, the product of an obstinate misrepresentation of sources. The author' misunderstanding of the technology of Zheng He's ships impels him to depict voyages no captain would attempt and no mariner could survive, including a 4,000-mile excursion along the Arctic circle and circumnavigation of the Pacific after having already sailed more than 42,000 miles from China to West Africa, South America, Australia, New Zealand, and the Philippines.

The review is worth the read, but the long and short of it is no, 1421 should not be considered as a reliable historical work.