Here is perhaps one of the best short overviews of Luther's character I've ever come across:
There is a great deal about Luther's character and history to call forth admiration and love; while there is also a good deal about him to afford an excuse to those who, from whatever cause, whether as papists or on some other ground, are disposed to regard him with opposite feelings. With many high and noble endowments, both from nature and grace, both of head and heart, which in many respects fitted him admirably for the great work to which he was called, and the important services which he rendered to the church and the world, there were some shortcomings and drawbacks both about his understanding and his temperament; the results and manifestations of which have afforded many plausible handles to his enemies, and have occasioned corresponding annoyance and difficulty to his friends.
Luther occupied a position, and exerted an influence in the history of the church, and altogether manifested a character, well fitted to secure for him the admiration of all who are interested in the advancement of Christian truth, or qualified to appreciate what is noble, magnanimous, fearless, and disinterested. We have abundant evidence of his continuing to retain the common infirmities of human nature, aggravated in some respects by the system in which he had been originally educated, by the condition of society in the age and country in which he lived, and the influences to which, after he commenced the work of Reformation, he was subjected; but we have also the most satisfactory evidence of his deep piety, of his thorough devotedness to God's service, of his habitual walking with God, and living by faith in the promises of His word. No one who surveys Luther's history and writings, and who is capable of forming an estimate of what piety is, can entertain any doubt upon this point.
The leading service which Luther was qualified and enabled to render to the church, in a theological point of view, was the unfolding and establishing the great doctrine of justification, which for many ages had been grossly corrupted and perverted; and bringing the truth upon this subject to bear upon the exposure of many of the abuses, both in theory and practice, that prevailed in the Church of Rome. His engrossment, to a large extent, with this great doctrine, combined with the peculiar character of his mind, led him to view almost every topic chiefly, if not exclusively, in its relation to forgiveness and peace of conscience, to grace and merit; and thus fostered a certain tendency to exaggeration and extravagance in his doctrinal statements. Besides this defect in Luther's theology, giving it something of one-sidedness, he had some features of character which detract from the weight of his statements, and from the deference to which otherwise he might have appeared entitled, and which we feel disposed to accord to such a man as Calvin. He was naturally somewhat prone to indulge in exaggerated and paradoxical statements, to press points too far, and to express them in unnecessarily strong and repulsive terms. And this tendency he sometimes manifests not only in speaking of men and actions, but even in theological discussions. He was not characterised by that exact balance of all the mental powers, by that just and accurate perception of the whole relations and true importance of things, and by that power of carefully and precisely embodying in words just what he himself had deliberately concluded, and nothing more, which, in some men, have so strong a tendency to persuade us to give ourselves up to their guidance, under a sort of intuitive conviction that they will not lead us often or far astray from the paths of truth. In Luther's works, with a great deal to admire, to interest and impress, we often stumble upon statements which remind us that we must be on our guard, that we must exercise our own judgment, and not follow him blindly wherever he may choose to lead us. The leading defects of his character may be said to be,— 1st, The impetuosity of his temperament, leading often to the use of exaggerated and internperate language, both in conversation and in writing; though, as has been frequently and truly remarked, very seldom leading him into injudicious or imprudent actions, amid all the difficulties in which he was involved : and, 2d, A certain species of presumption or self-confidence, which, putting on the garb of better and higher principles, sometimes made him adhere with great obstinacy to erroneous opinions, shutting his understanding against everything that could be brought forward in opposition to them; and made him indulge sometimes in rather ridiculous boasting. The result of all these qualities was, that he has left many statements of an intemperate and exaggerated description; which have afforded a great handle to his enemies, and which, when collected and set off by being presented in isolation from accompanying statements and circumstances, and in combination with each other, are apt to produce a somewhat uncomfortable impression.
And then consider how this extraordinary man, of so peculiar a mental character and general temperament, was tried and tested. He occupied a very singular position, and was subjected to very peculiar influences. He was tried in a very unusual measure, with almost everything fitted to disturb and pervert, to elevate and to depress, with fears and hopes, with dangers and successes. Let it be further remembered, that of this man, who was so constituted and so circumstanced, there have been preserved and published no fewer than about 2300 letters, many of them private and confidenrial effusions to his friends; and that a great deal of his ordinary conversation or table talk has been recorded and transmitted to us, without our having any good evidence of its being accurately reported.
It is surely not to be wondered at that it should be easy to produce many rash, extravagant, inconsistent, and indefensible sayings of Luther. And if, notwithstanding the tests to which he has been subjected, he still stands out as unquestionably a man of high religious principle, of thorough and disinterested devotedness to God's service, and of many noble and elevated qualities,—all which most even of his depredators, except the Popish section of them, will probably concede,—how thoroughly base and despicable is it in any man to be grasping at opportunities of trying to damage his character and influence, by collecting and stringing together (perhaps exaggerating and distorting), his rash and inconsistent, or it may be extravagant and offensive, sentiments and expressions. Papists, of course, are labouring in their proper vocation in trying, per fas aut nefas, to damage Luther's character. Popish controversialists are ever ready to sacrifice conscience, and every manly and honourable feeling, to the interests of the church ; and Tractarians, following in their footsteps, have imbibed a large portion of their spirit.