Ah, don't get me started with Justinian, such a controversial figure, so much to say about him. He had his pros and his cons like most emperors.
Administration:
Pros:
Ministers such as Tribonian and John of Capadoccia did much to improve the legal and financial systems of the empire, respectively. In the case of the former, his reform of the legal system, it must not be forgotten, was groundbreaking, and not only influenced Byzantium but also Europe and the modern world very deeply, it needs not be discussed here. The latter curtailed the power of local nobles in the provinces by taxing them as heavily as their subjects and left the tax-collection system of the empire better and more efficient than he found it.
Cons:
The negative side of the coin is that these two ministers were also famous for their corruption, and they were not the only ones, indeed, during Justinian, corruption became endemic within the empire and often magistrates would not be selected by their merit or virtue and those in high positions would often lavishly waste state money on gratyfing their own sordid pleasures, at the expense of the people. In fact, this corruption and rapine was what caused the famous Nika riots in the first place, and the rebels were in fact calling for the dismissal of the two aforementioned functionaries.
Reconquests:
Cons:
The corruption of his officials had terrible consequences in Italy, which suffered the most from the depredations of not only the rapacious logothetes and tax-collectors of Justinian, but also of the soldiery, factors which profoundly impoverished and diminished the productivity and happiness of the Italians, to the extent that they resented Byzantine more than “barbarian” rule. It must also be considered that the Goths were acceptable allies who were under nominal suzerainty of and friendly towards Byzantium, effective against invaders, and fair to the Italians, for example. If he was to replace this monarchy, he should have at least have ensured he and his successors could do a better job than them running the place. The administration he set up on the new territories was rather inadequate as well because it lacked the appropiate military character that was necessary to protect them, thankfully, Maurice was sensible enough to fix this with the exarchate. All in all, the re-establishment of Byzantine rule in Italy was costly, ephemeral, detrimental to Italy’s productivity (and therefore, trade), and in general, a disaster to everyone.
Pros:
He was more successful in Africa, where he rebuilt Carthage and restored the economy of the area, providing the empire with a rich and productive province.
Defence:
Pros:
He established countless fortresses in Asia and Europe to guard his territories, he swelled the numbers of the armies and also managed to contain the threat of one of the most formidable Persian monarchs, Anushirvan, though not without setbacks (Antioch was even sacked at some point).
Cons:
He let the fortresses deteriorate and the army diminish late in his reign, and the eastern provinces suffered heavily due to the raids of Chosroes, which were in part, a response to his successes in the West. The vulnerability of the empire's core was also exposed with the incursions of Slavs and Kortrigurs that took place late in his reign, as well. Finally, he overstretched the borders of the empire with the conquest of Italy, which made it harder to defend while draining resources.
Economy :
Pros:
During his reign trade flourished, and the Eastern provinces in particular enjoyed prosperity, new trade routes were encouraged through the Black Sea and through the Caucasus and towards Soghdiana to bypass Persian-controlled trade routes of the Silk Road, and, more importantly, Constantinople acquired her own silk industry, a monopoly which would be highly profitable and beneficial to the state in the future.
Cons:
He basically left the state bankrupt through his campaigning and the corruption and lack of restraint of his court and functionaries.
He deepened the rift between the many faiths within the empire, never a good thing in Byzantine politics. His paranoia in regards to Belisarius was also the root of many inefficiencies and setbacks during the man's campaigns as well. His suppression of the school of Athens is something which I frown upon, even though it did not have negative practical consequences to the state itself, and the institution had long declined anyway. By the end of his reign, his lost his energy and started resorting to buying off his enemies rather than fighting (as he did with Chosroes) and let the army and fortresses deteriorate, as I have mentioned. By this point it is clear he should have abdicated, but instead he damaged the empire by clinging on to power. He did, however establish plenty of public works and services, and his artistic and architectural legacy is unquestioned.
When he died the empire had extended its borders and contained the threat of one of Persia's greatest monarchs, but was also left financially exhausted and faced with the responsibility of new territories which had to be defended, was it all worth it? Well, if his successors had consolidated their position in Italy and maintained their authority there until productivity was recovered, perhaps. But since they didn't, his conquest of Italy was, with the benefit of hindsight, a wasteful and unnecessary expedition. All in all, he is hard to assess, that he worked in excess for the good of the empire is out of the question, but how beneficial his reign in general was to the Empire in the big picture is a more complex question which requires short-term and long-term analysis and consideration must be given not to one but to the many areas in which he left his imprint. My personal assesstment is ambiguous, but I wouldn’t call him the greatest of Byzantium by any means if perhaps the most dynamic, and I admit that his weakness towards his wife and his ingratitude towards Belisarius makes him abhorrent to me on a personal level.