That stats are from my developing board that hasn't much traffic so.
Here are some stats from my production board.
Uptime witch cache ~ 3h
select statements total 138.337
Qcache queries in cache 2935
Qcache inserts 137368
Qcache hits 153421
So it has very little effect. It has no effect on the main tables that cause the heavy load (posts, messages, users).
The small tables that have static content like categories, addressbook and smilies can be cached, but that should have much effect, cause this tables are normaly very small.
