It's pretty nice and useful. Especially if I wanna check quickly if there's any graphical error. I just wished the viewport on the right was as flexible as the one for the Image Attributes Tool (where you could zoom/dezoom/move around the sprite). As of now it's displaying the sprite twice, which can waste quite a bit of space (especially if you're trying to see the whole body). Maybe only keep one view (where you can zoom/dezoom) would be nicer.
Also personal preference but I'd rather when I click on a group, it replaces everything with the current group, than adding it to the current stack (maybe if you right click it, it could have this additional functionality?). Because I have a lot of attribute so I end up hiding/showing everytime I want to see a different group.