Re: What is good ukemi?
Great inquiry. We have had several Sensei's visit our little dojo in the past couple of years, and the question I always ask by the second beer Friday night is: "In your opinion, what is really good ukemi?" I have recieved some good explanations and even better physical examples by our visiting teachers.
I must say, however, that different teachers seem to appreciate different approaches. Some want an uke who is very responsive and bails in huge breakfall right away, others want an uke who will hang in there a little and is not afraid to scrap before taking a roll or falling backward in a good defensive ground position.
Most want an accurate attack that will land if no action is taken by nage. The ability to remain balanced throughout the attack and subsequent roll or fall is also pretty universely applauded.
I do sense some large differences of opinions in breakfalls (i.e. big flipping high falls off the ground with the huge slap... ya know, the one that impresses the audience), in that some teachers like them, others find them useless, and others just don't care as long as you can get up afterward.
To give an opinion on one or two of your questions, I think that we take ukemi for ourselves, in that it is a defensive move in order to save ourselves from the full brunt the technique (if it has the ability and application to truly do some damage). I think great ukemi involves being able to stay connected to nage in a way that one can reverse at any given opening in the martial exchange, even when moving to save oneself from technique (whether rolling, falling backward, or preparing for a breakfall). Good aikido technique, then, is that which is not open to reversal in its execution. Good ukemi should lead to good aikido technique, and vice versa.