The Atoma Diamond plate #140 grit is the gold standard for whetstone flattening. It removes material extremely quickly and allows you to flatten your stones with relative ease. While other synthetic truing stones will abrade and wear over time they will tend to lose their flatness especially quickly when truing exceptionally wonky stones. The Atoma in comparison is mounted on a solid aluminum block that will remain flat through all the highs and lows you throw at it!
They are also great for repairs as they can very quickly remove steel from a knife. We wouldn't make a habit of using it to sharpen knives though as doing so will pull diamonds off the surface making it less and less effective as a flattening or sharpening tool. Should you wear out your diamond plate, you can peel off the diamond pad and replace it down the road when that time comes.
#140 is quite coarse and will leave DEEP scratches when thinning, it is not recommended to thin a knife unless you have an adequate progression of stones to work out those scratches.
It is sometimes stated that the #140 will leave the surface of your stone too rough, though unless working on razors, we've not noticed it to be an issue, nothing a quick kiss with a nagura won't fix if you are though!
