Not a query cache so much as a document cache. I'm actually implementing a query cache in Redis which is, imo, the greatest tool on earth :P
To be honest the decision to use mongo as a document cache was made before I realized the scope of our data needs. Mongo was the quickest/easiest solution at the time (early 2012). If I could do this over again, and we actually will be in the next 6 months, I'd either set it up in PostgreSQL/json, or throw everything in a wide column store.