Some of my sempai can take ukemi in a manner that makes me perform the technique properly. This is amazing to me.
But I also like when they attack with an 'empty mind', and just go where the energy takes them.
Then again, sometimes I also like when they attack with the intent of foiling my technique.
Sometimes at work I'll be having a conversation with a customer, and one of my coworkers will, picking up on something pertinant, go and grab a product or do something for them while we're still talking, before I even start to move. It's very aiki. This is my favorite associate to work with.
Oh, so anyhow, it seems to me that good ukemi is very much like good waza, and the two go together like a tag team.