TL;DR
If one adheres to deontological or rule-utilitarian views, it is probably completely immoral. Other views on morality may say that the means justify the ends. Taking The Prince seriously contentwise (not everybody does, see e.g. Garrett Mattingly, who thinks it is sophisticated satire), one has to conclude that Machiavelli himself thought that because of the political reality, immoral deeds are inevitable for achieving a morally acceptable society and political environment. One could say: You have to play by the (immoral) rules.
Long story
I will base my answer completely on the comparatively recent book:
Viroli, M. (2013). Redeeming "The Prince": The Meaning of Machiavelli's Masterpiece. Princeton University Press.
Viroli's main thesis throughout the book is that the twist of the last chapter showed that although there may be immoral action throughout the reign of the prince, the ultimate role is that of a redeemer, of instantiating utopian ideals as real in a time of political chaos:
What then is The Prince about, and what is its lasting
value, if any? My answer is that Niccolò Machiavelli wrote
The Prince to design and invoke a redeemer of Italy capable
of creating, with God’s help, new and good political order,
thereby attaining perennial glory. The theory, and the myth,
of the redeemer is, in my opinion, the enduring value of
Machiavelli’s little book. As I shall document in the last
chapter of this essay, the interpretation of The Prince as a
discourse on political redemption has a long and fascinating
history. Yet contemporary scholarship, with a few exceptions,
has instead disregarded or dismissed it. (p.3)
It is, therefore, both moral and immoral: While the political activity cannot be completely detached from morality (see e.g. pp. 1-3, 18, 114), the necessary political action of a prince (or, as Viroli speaks of "enduring value", probably any political actor) may and has to be justified by the achievement for a general betterment of society, i.e. an overall morally superiour status:
Only if we interpret Machiavelli’s prince as a
founder and redeemer, Villari noted in the conclusion of his
work, can we understand and justify his immoral counsels:
But when, completing his analysis, and cruel labour of vivisection,
Machiavelli proceeds to draw his conclusions, then
at last the practical side and real aim of his work are clearly
seen. It is a question of achieving the unity of his Italian
motherland and of delivering it from foreign rule. This was
certainly the holiest of objects; but Machiavelli well knew
that in the conditions in which Italy and Europe were then
involved it would be impossible to achieve that object without
recurring to the immoral means practiced by the statesmen
of his time. Pursued by this idea, and dominated by
this theme, Machiavelli did not pause to disentangle the
scientific, general and permanent aim of his book from the
practical aim and transitory means, apparently and, it may
be, really essential to its achievement at that moment. It is
needful, he said in the conclusion, to dare all things, and in
view of the grandeur and sacredness of the end, to yield to
no scruples. Solely by the formation of a united, powerful,
and independent nation can Italy acquire liberty, virtue and
true morality. This is an enterprise only to be undertaken by
a Prince–reformer, and by means suggested and imposed by
history and experience. The people must afterwards complete
and consolidate it by liberty, by national arms, by public
and private virtue. (pp. 176-7 note 38, citing Pasquale Villari, Niccolò Machiavelli e i suoi tempi; English
translation by Madame Linda Villari, The Life and Times of
Niccolò Machiavelli, London, T. Fisher Unwin, 1892, vol. I, p. 196)
One could, therefore, conclude that while applying the standards of deontological morals it is a completely immoral book, according to certain utilitarian views the means justify the end, just as Machiavelli himself may have seen it (which surely is Viroli's view).
All in all, every political actor has to be judged by moral standards and Macciavelli does argue this as well. In this sense, The Prince surely is not immoral. All Macciavelli does is opening the scope from individual actions to "the whole reign will be judged by history", which is actually, as Viroli argues, an enduring picture still influential in modern history, and probably the reason why the book is part of the curriculum of (at least most of the) ivy league colleges educating the soon-to-be powerful.