Re: What is good ukemi?
IMHO it depends on the context. In training good ukemi is (IMO) is being aware of what tori is doing, what is happening around you, not going anywhere you've not been put and not falling over unless you have to. Basically simulating someone who's balance has been broken by a previous technique.
Good ukemi in a live situation, for me anyway, is actively moving from an unfavorable position to a more favorable one, using tori's movement and power to achieve this.