You can load custom ROMs (i haven't checked out the app, cause I don't have an evo, it should have a custom recovery image) by doing exactly what you said, load it up in recovery mode, make a nandroid backup, wipe data (put the .zip file as is in the root of the sd card), flash .zip. If you don't like the ROM, nandroid restore back to as it was before the flash, 100%, no messed up data.
I don't see a thanks button but thanks