Here is an excerpt of how the 4th century historian Ammianus Marcellinus eulogized him:
In taking care of the soldiers he was very cautious: an examiner into their merits, sometimes over-scrupulous, giving dignities about the palace as if with scales. Under him no one who was not well known to him, or who was favored merely by some sudden impulse, ever received any high appointment in the palace. But only such as had served ten years in some capacity or other could look for such appointments as master of the ceremonies or treasurer....
He was a diligent cultivator of learning, but, as his blunted talent was not suited to rhetoric, he devoted himself to versification; in which, however, he did nothing worth speaking of. In his way of life he was economical and temperate, and by moderation in eating and drinking he preserved such robust health that he was rarely ill, though when ill dangerously so....
He was contented with very little sleep, which he took when time and season allowed; and throughout his long life he was so extremely chaste that no suspicion was ever cast on him in this respect, though it is a charge which, even when it can find no ground, malignity is apt to fasten on princes.
In riding and throwing the javelin, in shooting with the bow, and in all the accomplishments of military exercises, he was admirably skilful. That he never blew his nose in public, never spat, never was seen to change countenance, and that he never in all his life ate any fruit I pass over, as what has been often related before.
Having now briefly enumerated his good qualities with which we have been able to become acquainted, let us now proceed to speak of his vices. In other respects he was equal to average princes, but if he had the slightest reason (even if founded on wholly false information) for suspecting any one of aiming at supreme power, he would at once institute the most rigorous inquiry, trampling down right and wrong alike, and outdo the cruelty of Caligula, Domitian, or Commodus, whose barbarity he rivaled at the very beginning of his reign, when he shamefully put to death his own connections and relations
And his cruelty and morose suspicions, which were directed against everything of the kind, were a cruel addition to the sufferings of the unhappy persons who were accused of sedition or treason....
In such cases he had a mortal hatred of justice, even though his great object was to be accounted just and merciful: and as sparks flying from a dry wood, by a mere breath of wind are sometimes carried on with unrestrained course to the danger of the country villages around, so he also from the most trivial causes kindled heaps of evils....And, as some right-thinking people are of opinion, it was rather an indication of great virtue in Constantius to have quelled the empire without shedding more blood, than to have revenged himself with such cruelty....
But as in his foreign wars this emperor was unsuccessful and unfortunate, on the other hand in his civil contests he was successful; and in all those domestic calamities he covered himself with the horrid blood of the enemies of the republic and of himself; and yielding to his elation at these triumphs in a way neither right nor usual, he erected at a vast expense triumphal arches in Gaul and the two Pannonias, to record his triumphs over his own provinces; engraving on them the titles of his exploits ... as long as they should last, to those who read the inscriptions.
He was preposterously addicted to listening to his wives, and to the thin voices of his eunuchs, and some of his courtiers, who applauded all his words, and watched everything he said, whether in approval or disapproval, in order to agree with it.
The misery of these times was further increased by the insatiable covetousness of his tax-collectors, who brought him more odium than money; and to many persons this seemed the more intolerable, because he never listened to any excuse, never took any measures for relief of the provinces when oppressed by the multiplicity of taxes and imposts; and in addition to all this he was very apt to take back any exemptions which he had granted.
He confused the Christian religion, which is plain and simple, with old women's superstitions; in investigating which he preferred perplexing himself to settling its questions with dignity, so that he excited much dissension; which he further encouraged by diffuse wordy explanations: he ruined the establishment of public conveyances by devoting them to the service of crowds of priests, who went to and fro to different synods, as they call the meetings at which they endeavor to settle everything according to their own fancy.
As to his personal appearance and stature, he was of a dark complexion with prominent eyes; of keen sight, soft hair, with his cheeks carefully shaved, and bright looking. From his waist to his neck he was rather long, his legs were very short and crooked, which made him a good leaper and runner.
Read the full account here: http://www.tertullian.org/fathers/ammianus_21_book21.htm#C16