Last night, I received "Aikido Shugyo" by Gozo Shioda in the mail and started reading it. It is an incredible account of initial study of aikido and I really gathered (only half way through the book) that Timing... is everything.
Sensei Shioda addresses how people attack, whether they plant their feet or not, and gives ideas on how to use such an attack to your advantage.
Like I said, I'm only half way through the book and I have no intention of relaying the whole book in a message here. Suffice to say, timing is very critical but no matter how they attack, committed or not... planted or not, he addresses it well and gives the reader much to think about.
Very recommended reading, imho.