The sports med. program in my school (I'm a math teacher, not a student!) has a hand held electro-thingy... You enter in gender, age, height, weight, choose "normal" or "athlete" and hold on to the handles.
I have no idea how consistant it is, but it has always been within 1.5% of a caliper reading (checked twice- not every time with calipers).
I figue that if I use that thing at the same time of day, after eating and drinking pretty much the same thing, after using the rest room, etc., that in relative terms, it is useful for tracking progress.