I have no problem with a mix of CCM and Hymns. I think most of the hymns that I am familiar with are better and more God glorifying than most CCM, but that is not to say all hymns are good or all CCM is bad.
Where I have a big issue is when a church replaces all of the hymns with CCM and puts a "praise band" instead of piano and organ, so now its much more akin to a rock concert than a worship service. I find it very hard to get my heart in tune with worshipping the Lord with 2 guitars, electric bass, electric keyboard, full drum set all heavily amplified.
Worship should come from the heart. It is more than the words we sing, or the music that is played. Music can and does have a big influence on how we feel at a given time. Hollywood knows this well. Ever listen to a movie soundtrack? The music foretells what is coming in the scene. It can be foreboding, happy, joyful, sad, etc. It influences mood and emotional content. You might immediately dislike a character in a movie based on the music that accompanies that characters appearing because it brings an association with it.
The key a piece of music is in, the timing of it, the volume of it, even the instrumentation all is part of this. Why do we think that we can import a rock beat and instrumentation, timing, volume and get worship out of it by only changing the words?
We are worshipping the God that made the universe, who from His loftiness, had mercy upon a people who He is carving out for Himself among all those who ever walked the earth. Absolutely there should be celebration in this, but there also needs to be referent reflection on our unworthiness before Him. An opportunity to revel in His love.
Maybe this is an extreme of CCM that most churches don't have in it, I just know it does nothing for me, and actually interferes with my ability to properly worship.