You have to hit solo with your daily - probably like 40% chance ?
It has to fail a save (solos have +5 to saves). 20% chance.
So, basically you are blowing up your daily spell to have 8% chance of putting the solo to sleep for 1 round. 1.5% chance for putting him to sleep for 2 rounds. And sleeping doesn't mean automatic kill - people still have to hit the monster (albeit with +5 modifier) and they deal auto crit. Critical is probably comparable to second attack on top of basic one(a lot worse on low levels, bit better on high levels), so with 5 person party, you have 8% to deal +5 attacks damage. At the cost of your daily spell.
I think it is completely in line with other possibilities - compare it to warlord daily, which can give +5 to hit for everybody till end of encounter (and you just have to hit, you don't have to hope for the save failure). THAT one is an overkill against solos.
Sleep power is really visible if you use it against the group of relative weenies. You have better chance to hit, they have reasonable chance to fail their saves.
Now, if you use the orb abilities and have very high wisdom, suddenly your chance of getting job done properly increases dramatically... but I feel it is more a problem of orb ability rather than sleep itself.