Sheeri explains why turning on the query cache without thinking about the consequences is bad but also why simple benchmarks proving 'the query cache is bad' may be skewed

Maximize your strengths, minimize your weaknesses. You can apply this approach to many things in life, I apply it to describing and using MySQL the product, and it’s components. The Query Cache like many features in MySQL, and indeed features in many different RDBMS products (don’t get me started on Oracle *features*) have relative benefits. In one context it can be seen as ineffective, or even detrimental to your performance, however it’s course grain nature makes it both trivial to disable dynamically (SET GLOBAL query_cache_size=0;), and also easy to get basic statistics on current performance (SHOW GLOBAL STATUS LIKE ‘QCache%’;) to determine effectiveness and action appropriately.

Mark Callaghan tests the query cache. "The query cache has had an interesting history with MySQL. I don't have much experience with it. I might want to use it in the future so I enabled it during a run of sysbench readonly on an 8-core server. The results are fascinating."