Although I prefer contemporary renderings both of prayer books, and of Scripture in the language of our speaking and thinking, I love the old language of Cranmer, who was skilled in his use of the English of that day. There is a beauty in the simplicity and grandeur of the language in this book. But most importantly, the truth of God in Christ and how that applies to us wonderfully resonates through the prayers and liturgy in it.
Of course I believe in sponataneous prayers with our own words, and in prayers referred to in 1 Corinthians 14. But I agree with Lauren Winner that prayer books help keep God in the center of our prayers. Ann Spangler and Lois Tverberg's edifying book, Sitting at the Feet of Rabbi Jesus, helps us understand the Jewish roots of the Christian tradition of liturgy and prayer books. I believe without a doubt that using them can help us immensely. I've believed this for some time, but have not really applied it well. But I'm growing especially in the conviction that this is true, and some in the practice. Scot McKnight's book, Praying with the Church, is an excellent introduction to prayer books and their use.
Jesus taught his disciples to pray the "Our Father prayer" (also here), so that this should be an important part of our prayer lives. Set prayers do help us keep on track. It's so easy to pray in ways that can end up self-centered, or being concerned about what makes people happy, more than God's will.
Again, liturgical praying should not be the only praying we do. A considered look at the psalms should put that to rest. But the words of liturgical prayers can begin to seep in our hearts and change our prayers and our lives.
Here are two prayers from The First English Prayer Book (p 113-114):
Almighty and everlasting God, which hatest nothing that thou hast made, and dost forgive the sins of all them that be penitent: create and make in us new and contrite hearts, that we worthily lamenting our sins, and acknowledging our wretchedness, may obtain of thee, the God of all mercy, perfect remission and forgiveness; through Jesus Christ our Lord. Amen.
Almighty God, which dost see that we have no power of ourselves to help ourselves: keep thou us both outwardly in our bodies, and inwardly in our souls, that we may be defended from all adversities which may happen to the body, and from all evil thoughts which may assault and hurt the soul; through Jesus our Lord. Amen.