One of the frequent criticisms I hear about Orthodoxy or any so-called "High Church" Christian confession, whether Roman Catholic or Lutheran or Anglican is that the people pray prayers written by other people. In other words, the prayers said during Liturgy or during the offices lack the spontaneity of true devotion and thus are somehow not as well received by God as compared to something that comes from a few seconds' thought. Ridiculous!
I've been to many church services where all the prayers were of the spontaneous variety and, honestly, they all sounded the same. Now, I do not doubt, in any way, the intentions of the persons praying them, but isn't there so much more than just to be "thankful for being in Your presence" etc.? The added over emotionalism of trying to generate tears to go with the spontaneous prayer only makes the person prayer look ridiculous. Again, I do not doubt the intentions of such people, but they will insist that God wants us to pray like that and that it will be more received than some dry prayer written by someone 300, 500, 1000 or even 2500 years ago. Some of these very same people will even refuse to say the Lord's prayer, going so far as to call it a "vain repetition" which is a gross abuse of that passage in Scripture.
Written prayers are not bad nor is their use a sign of spiritual weakness. The key is to move the prayers from the lips to the mind to the heart. Once prayer moves to the heart and the heart prays unceasingly, then it truly becomes your own. Yes, I pray prayers of St. Basil, St. John Damascene, St. John Chyrsostom, St. Ephraim, etc.. I also pray the Psalter, which is God's given prayer-book to us. I also pray the prayer that our Lord gave to us. I didn't author them but they are truly mine because I have fused them to my heart.
There is also humility in praying like this. It demonstrates that I am at a loss at what to say to God, but the words of Scripture and the Fathers of the Church are my guide to communion with God through prayer. For many years I was unable to pray. When I discovered these prayers, I finally found the words I was looking for all along and made them my own.
Ex tempore or spontaneous prayer isn't bad, but what more spiritual benefit does it impart? It doesn't give anything more. I will close with these words from an Anglican from 1649:
No man can assure me that the words of his ex tempore prayer are the words of the holy Spirit: it is not reason nor modesty to expect such immediate assistances to so little purpose, he having supplied us with abilities more then enough to expresse our desires aliundè, otherwise then by immediate dictate; But if we will take David’s Psalter, or the other Hymnes of holy Scripture, or any of the Prayers which are respersed over the Bible, we are sure enough that they are the words of Gods spirit, mediately or immediately, by way of infusion or extasie, by vision, or at least by ordinary assistance. And now then, what greater confidence can any man have for the excellency of his prayers, and the probability of their being accepted, then when he prayes his Psalter, or the Lords Prayer, or any other office which he finds consigned in Scripture? When Gods spirit stirres us up to an actuall devotion, and then we use the matter he hath described and taught, and the very words which Christ & Christs spirit, and the Apostles, and other persons, full of the Holy Ghost did use; If in the world there be any praying with the Spirit (I meane, in vocall prayer) this is it.
Jeremy Taylor, An Apology for authorized and set forms of Liturgy against the Pretence of the Spirit (1649).