Speaking of covers, take note that the image processing is done with 32-bit color depth and that you can generate thumbnails and resize images with the embedded Lanczos algorithm. The app supports 49 predefined metadata fields, limitless custom fields and a wide variety of images (for the cover art).
As you already hinted, you can edit all tags and add custom ones at the same time. The highlight of the program is the enhanced editor where you can check out and compare the existing metadata with the one from online databases. The interface is minimalistic and enables you to browse the music via a Windows Explorer style viewer. The idea behind the application is to enable you to preview the metadata of your audio samples and edit it accordingly, if necessary.
Inspect and, if necessary, change the metadata information
EZ Meta Tag Editor is a lightweight piece of software that enables you to browse the file information and modify the metadata of various tracks and samples you mixed yourself.