I've never tried writing a data formatter, but custom summary formats hardly ever work, so much so that I never even bother trying to use them any more. You can get them to work once just to tantalize you and cause you to waste loads of time, but then they never work again. I just use "View as Memory" or use the debugger console to print stuff out. The Expressions window would be potentially useful here but it's just too annoying - half the time you get the dreaded "Out of scope" and if you mistype, you can't select an existing expression and edit it, you have to type it all over again. In the console you've got editing built in so it's much easier.