I use my Expo Disc on a regular basis. I find it to be fairly good in most situations. Where light temps. vary from one spot to the next such as in auditoriums, gyms, etc., it is only as good as the spot where you measure. As an example I used it while photographing a live performance of a choir and where one person would be perfect a few feet away they would look yellow. The Expo Disc is not a cure all but will go a long way towards getting WB correct.
Reason I ask is that on 2 occassions my WB has been way out and to be honest I did not know at the time that it was such a big issue.
One, was when photographing dragonflies over a pool with a lot of reflected light and the other was taking birds on a dull day in the garden.
Mind you now I only shoot in RAW then I will be able to compensate, but I rather get it right first time round.