At the end of the Book of Job the LORD (twice) says to Job’s three friends:
לא דברתם אלי נכונה כעבדי איוב
“You have not spoken the truth about me as did my servant Job” (42:7-8 JPS), or, as some translations put it, “you have not spoken well of me”.
In a comment here Jen asked a very good question: “Does Job speak well of God in his confessions (40:3-5 and 42:2-6), or is it of his speeches in general that God tells the friends that Job was correct in his presentation of God?”
The comment addressed to the friends “You have not spoken the truth about me” could only refer to their speeches during the dialogue with Job, so I would be inclined to think that the words “as did my servant Job” referred to Job’s speeches during the same dialogue, and not to his final brief response to the LORD. I agree with Norman Habel  when he says “The blunt and forthright accusations of Job from the depths of his agony are closer to the truth than the conventional unquestioning pronouncements of the friends … Job’s answers correspond with reality. They are devoid of dissembling and flattery”.
This reminded me of something I’d read in Eugene Peterson’s Introduction to Psalms :
‘Untutored, we tend to think that prayer is what good people do when they are doing their best. It is not. Inexperienced, we suppose that there must be an “insider” language that must be acquired before God takes us seriously in our prayer. There is not. Prayer is elemental, not advanced, language. It is the means by which our language becomes honest, true and personal in response to God. It is the means by which we get everything in our lves out in the open before God …
‘In English translation, the Psalms often sound smooth and polished, sonorous with Elizabethan rhythms and diction. As literature, they are beyond compare. But as prayer, as the utterances of men and women passionate for God in moments of anger and praise and lament, these translations miss something. Gramatically, they are accurate. The scholarship undergirding the translations is superb and devout. But as prayers they are not quite right. The Psalms in Hebrew are earthy and rough. They are are not genteel. They are not the prayers of nice people, couch in cultured language.’
Peterson went on to encourage ‘raw honesty and detailed thoroughness in our praying’ and I am convinced that this is how the Book of Job encourages us to approach God. Not with carefully worked out theological ‘truths’, but with raw honesty, articulating our despair, anger, disappointment and frustration. To speak well of God is to challenge him when his world appears to be unfair and his ways unjust.
David Wolfers  came to this conclusion about how Job spoke the truth concerning God:
‘Job has penetrated to the truth about the moral conduct of the world, that the quality of an individual’s life is unrelated to his moral deserts; that disaster is a random occurrence as likely to befall the righteous as the wicked; that God does reject the innocent and reward the wicked as individuals as aften as He does the reverse. What Eliphaz and his friends have maintained, from 4:7 … to 20:29 … is sentimental rubbish, at odds with all experience of life.’
As a slight digression, Habel  makes this interesting observation about Job’s priestly role in acting as mediator for his friends:
‘Job is reinstated as mediator even before his family and possessions are restored. He is again to act as a patriarchal intercessor like Abraham (Gen 18:23ff.). Job had previously looked for a friend who would support him against God if necessary (6:14), an arbiter who would handle his case with God (9:33), an advocate who would defend his suit with God (16:19-20), and a redeemer to vindicate him after his death (19:25). But Job stood alone and achieved his own meeting with God. Now the one who sought a mediator becomes the mediator.’
So it seems to me that to ‘speak the truth concerning God’ is less about correct theology (the approach taken by the friends) and more about being honest, blunt if necessary, and being based in reality.
 Habel, N. C., The Book of Job: A Commentary (London: SCM, 1985), 583
 Peterson, E., Psalms (Colorado Springs: Navpress, 1994), 3f
 Wolfers, D., Deep Things Out of Darkness (Grand Rapids: Eerdmans, 1995), 462
 Habel, 584