You posed an interesting question, and got some very useful answers, so here's one that may be a bit provocative for all of us:
How much fun do the folks in each of the categories Matt listed think it is to train with each of us?
hard to say I think. I know I'm in that dangerous middle ground where I know just enough to begin offering advice, but not enough to feel very confident about it. I know I personally tend to "tank" a little more often than most of the folks in my dojo and that can be frustrating to someone who's pretty earnest about getting strong feedback.
This saturday's training for me was certainly filled with the nice feeling of getting a lot of experienced help with my waza. At times I felt a bit self-conscious that when my sempai were training, they got to really work on making technique work with greater experience. When I trained with them, the flow was often interupted. I didn't get the impression they were bugged by it in the least, but I think I saw a different level of enjoyment when they got to train with each other. So in both cases I think it was fun...just a different kind of fun.