In the past few years I have almost completely switched to a fluid design and mostly use rem/em, ch and % for dimensions and margin / paddings.
What I find a bit awkward when adjusting the value of margin and padding with the mouse is that em/rem and ch are only changed in whole values. This causes then usually quite large changes. In my opinion it would be better to adjust the values of some units (ch, em, rem, cm) in quarter steps, e.g. 0.25em.
Larger values could then still be set very easily with the mouse.
PS: It would be great if you could store the desired default units in the options, but I think that is already planned. Especially if fluid fonts are comming using em/rem is very useful I think.