For quite a while, I was avoiding the use of stun on my monsters, since I don't like to completely remove my player's actions. Finally, in a recent fight, I had a boss with one awesome daily power that inflicted Stun (Save Ends) on one target. I hit the party caster with it...
...who proceeded to fail 7 straight saving throws. (Both his natural ones, and every free save the group gave him.) Even with a +1 item bonus on all of them, and some extra bonuses on some of the other ones.
I would have felt worse if he hadn't previously been a Blood Mage who regularly dropped Destructive Salutation on everything in sight. But yeah, Stun should be handled with care.
More importantly, I realized that it is best used in fast-paced combats, where it doesn't take someone out of the fight for as long (in real time.) So it might be more useful with a Solo, where rounds go quickly - but in a fight with a dozen enemies, you probably want to avoid it.