Most attacks against Catholic prayer come from three fronts. First, I have heard complaints about the Eucharistic prayers (that the Priest says during the sacrifice of the Eucharist at every mass). Secondly, I have heard complaints about the Rosary - and the repetitiveness of the Hail Mary specifically. Third, people complain that we use written prayers instead of spontaneous prayers in so many cases. I want to deal with all these complaints together.
I challenge anyone to sincerely study the meaning and depth of the Eucharistic prayers or even the Hail Mary and try to tell me that they are simple, vain, and repititious. The Hail Mary, in and of itself, is so deep with scripture and theology that you could pray it a billion times and meditate on different aspects of it each time. No one can completely break down the prayer, no matter how simple it is and all the time we hear people who meditate upon it bring out some other form of depth from its midst that we have not heard before.
Also, let's discuss how the Lord asked us to pray. The Lord's Prayer is the beautiful and also rich prayer that Jesus taught his disciples and told them to pray. It has been part of the Church's worship since the first days of the Church. Again, he didn't tell us to pray it just so we could be "repetitious" but rather so that we could come to a deep feeling about the Church, our role in the world and our relationship with his Father.
Furthermore, complainers should do well to realize that scripture reading and hymn singing are both forms of prayer, and that they are written and could be considered just if not more "repetitive" then Catholic prayer.
Jewish prayer also (along with the Catholic Liturgy of the Hours) has always been grounded in the Psalms. These written prayers teach us much about how to pray and how to respect God. Considering that Jesus prayed the same prayer 3 times in the Garden of Gethsemane, and that Psalm 136 says "And his steadfast Love endures forever" 25 times, and that those surrounding God's throne sing "Holy Holy Holy" FOREVER, I'd say its a hard argument.
It's at least much better to pray a deep and thoughtful prayer over and over with different meditations than the same bland "spontaneous" prayers that most of us try to do every time we pray just because we aren't that creative or can't think up great new theological prayers on the spot.