I think that any single type of evaluation of WM is probably flawed in that it unfairly evaluates some people on the basis of an area of strength (I'd look... probably off the charts at a COLOR task... and most people wouldn't)... and others on the basis of an area of relative weakness (me with digits) due to differences in the nature of the information in the task.
This is a really good point. I scored right in the average range for digit span on the tests here
http://www.cambridgebrainsciences.com/user/login/, but abysmally low on the test for directional memory (spatial span). (No surprise. I could get lost in a cardboard box. I definitely have some kind of LD or something. It's quite humbling.) My DD did better than I did on the spatial memory test, but I think she was in the average range, perhaps even slightly below average, whereas she was above average (for an adult) for digit span. I'd never say I have a good short-term memory. Right now it's probably at its worst ever; I'm on a med that causes brain fog. The only thing I have an unusual memory for is spelling--I'm in the "see a word once" camp.
