I bought an ex-25 extension tube this week; since I shoot a LOT of macros it seemed like a good investment for me. I just received it yesterday so I haven't had the chance to play with it in the field but I did get a chance to run it through the range of focal lengths on my lenses, just to see how it behaves.
Here's a collage of absolute magnification, without the tube:
And with it:
The 14-54 is not able to focus until it reaches about 40mm, so for the sake of brevity I did not test it at anything other than 54mm.
The 50 f/2 macro is surprising because it is still possible to focus between 1x and 2x (estimated), which is a large range of magnification options - the other lenses don't have as much focus travel so they don't get nearly as wide a berth.
The 40-150 is a surprise as well, and because it has the greatest working distance and the largest zoom range, it will probably see some use with the tube. Magnification isn't great but it might be a good trade for working distance.
More as I actually use it.