I was interested reading this post because I've just bought a DST compressor. I think the answer as to whether it is useful depends on swing speed. If you swing fast, the club head mass will likely straighten the shaft completely, as mentioned and photographed.
However, that isn't how I use it. I use it for slow half swings to feel the position I need to be in with my hips rotating towards the target and my right arm slightly bent. It allows me to develop the correct position and then try to increase the length of backswing. Increasing length of backswing then increases the speed of the swing, but I try and keep that same feeling.
Of course, as mentioned there are other ways to do this, with alignment sticks etc, but I find this to be a really useful training aid. I've bought others which haven't had the same impact on my swing.
When I swing slow, I don't see the shaft straightening completely. See the pictures here, it is clearly bent even after impact. My camera only does 120fps, but I don't think that should mean it changes the flex of the shaft on the image?
The shot was a clean hit and had that lovely sound of a compressed ball strike which I didn't get very much before. When I switch back to my normal 8 iron and try and recreate the feel, I'm getting more compression. I've now started hitting my 9 iron 150+ yards, when I hit it correctly, whereas it was about 135-140 before. So I suppose those number don't lie, the training aid has done something for me. I'm having to club down as it is changing my yardages.