it may be because i'm small, and even for my size have small hands, but i usually find it easier to blend and lead, relying on timing rather than atemi. Atemi seems to stop ukes, who when they are big and heavy footed, as many of mine are, getting them moving again can be difficult. I do like to use it if i have to pass close to uke (as in sudori techniques) or if i am committed to a really fast straight in movement. Otherwise acute reality often points out the unlikelihood that my 100 pounds, even if exactly right on target to a vulnerable area (ha) is going to slow down my 200 pound uke. and since i usually find that big male ukes will throw their faces full speed into my outstretched hand, i am beginning to doubt the average American male is willing to get off balance just to save his face.
on the other hand, as i am more than willing to sacrifice my balance or speed of attack to save my face, it does seem quite effective when i work with a good sized nage. i guess, as in everything in Aikido, it just depends...