Here’s what Gibbon had to say:
His abilities were useful rather than splendid; a vigorous mind, improved by the experience and study of mankind; dexterity and application in business; a judicious mixture of liberality and economy, of mildness and rigor; profound dissimulation, under the disguise of military frankness; steadiness to pursue his ends; flexibility to vary his means; and, above all, the great art of submitting his own passions, as well as those of others, to the interest of his ambition, and of coloring his ambition with the most specious pretences of justice and public utility. Like Augustus, Diocletian may be considered as the founder of a new empire. Like the adopted son of Caesar, he was distinguished as a statesman rather than as a warrior; nor did either of those princes employ force, whenever their purpose could be effected by policy.