Procopius knew many of the people he was writing about firsthand; he served as a lawyer and military legal adviser/secretary to the famous general Belisarius, and later authored two official works for the Byzantine court; the History of the Wars, an eight volume account of Justinian’s campaigns, and On Buildings, an panegyric on Justinian’s building projects. The Anekdota or Secret History, was not published during his lifetime; the content of the text, as you no doubt noticed, is not exactly flattering to the regime.

The text is notable for containing a fair number of outlandish, supernatural accusations against the emperor Justinian and his wife the empress Theodora; the emperor is allegedly a demon in human form, and several portions of the text contain stories depicting supernatural behavior -- at one point, the emperor's head vanishes!

And some of those who have been with Justinian at the palace late at night, men who were pure of spirit, have thought they saw a strange demoniac form taking his place. One man said that the Emperor suddenly rose from his throne and walked about, and indeed he was never wont to remain sitting for long, and immediately Justinian's head vanished, while the rest of his body seemed to ebb and flow; whereat the beholder stood aghast and fearful, wondering if his eyes were deceiving him. But presently he perceived the vanished head filling out and joining the body again as strangely as it had left it.

Historians can safely disregard these supernatural claims; I think that goes largely without saying. However, there are other elements of the work that are also questionable, particularly those dealing with the empress Theodora, and the scandalous accounts of lurid, depraved behavior, including the infamous "geese routine."

Often, even in the theatre, in the sight of all the people, she removed her costume and stood nude in their midst, except for a girdle about the groin: not that she was abashed at revealing that, too, to the audience, but because there was a law against appearing altogether naked on the stage, without at least this much of a fig-leaf. Covered thus with a ribbon, she would sink down to the stage floor and recline on her back. Slaves to whom the duty was entrusted would then scatter grains of barley from above into the calyx of this passion flower, whence geese, trained for the purpose, would next pick the grains one by one with their bills and eat.

Procopius also implicates Theodora in the death of the Ostrogothic Queen Amalasuntha, when in reality she very likely had nothing to do with Amalasuntha's murder; she was killed on the orders of her cousin and co-ruler Theodahad, in large part because of her pro-Byzantine sympathies.

What of Justinian? Procopius does not paint a flattering picture of the emperor, who he depicts as cruel, incompetent, foolish, manipulative, and a whole host of other negative qualities, as well as implying that his rule (and by extension, the rule of his uncle, emperor Justin I) was illegitimate. Procopius attacks Justinian for wasteful building projects, changes to the legal codes, tax policy, military matters, and more; there's hardly a single decision that Justinian makes which Procopius approves of. Justinian made many sweeping changes to the empire during his reign, and Procopius was a traditionalist through-and-through; a careful reading of the work reveals a man deeply dissatisfied with the course the empire was taking.

Therein lies the value of the Secret History. As a lawyer by trade, Procopius would be intimately aware of Justinian’s revisions to the legal codes; as a secretary attached to Belisarius, he would be likewise be acutely aware of the state of the military. Procopius’s place in the inner circle of the Imperial Court at Constantinople would have allowed him insight into the financial policies of the regime, and put him in an almost unrivaled position to chronicle the intrigue and bureaucratic machinations at the Imperial Court.

Procopius also provides a valuable source for certain aspects of Byzantine daily life and culture; at one point in the text, he describes in detail the dress of the two rival Hippodrome factions, the Blues and the Greens, and at another he tells a story of Theodora and her courtiers mocking a nobleman about a hernia.

In short, is Procopius a straightforward source? No, not at all; he was clearly writing with an agenda. As much as the History of the Wars and On Buildings are panegyrics, the Secret History is an anti-panegyric, the outpourings of one deeply-disgruntled employee. However, careful reading of the text, combined with an understanding of what kind of man Procopius was -- a traditionalist, legalistic man -- allows historians to learn a great deal about the reign of Justinian. I'd say take his more outlandish claims with a grain of salt, particularly when relating to Justinian or Theodora, but he offers a great deal on the topic of law, military matters, bureaucracy, etc., etc.

Let me know if you'd like a more detailed follow-up answer; I'd be happy to cite more passages or go into more depth if need be.

Edit: Whoops, almost forgot my sources:

Procopius of Caesarea, The Secret History (available online here)

A History of the Byzantine State and Society, by Warren Treadgold

The Cambridge History of the Byzantine Empire c. 500-1492 edited by Jonathan Shepard (2008).