Own a set of DMT, Ultra Sharp, Atoma, and a Sharpal, among a few other of the dirt cheap no name diamond plates. Had good experiences with the Ultra Sharp and Sharpal, great experiences with the Atoma, mixed with the DMT.
The Ultra Sharp and Sharpal are more or less the same in quality, at least the ones I have. No grit contamination on either. Sharpal arguably a better value as you get two grits on their double sided plate, though Ultra Sharp has more grit selection, from 300-3000. Sharpal currently only offers the one double sided plate afaik. A 325/1200. The Sharpal 325 felt way more coarse than the Ultra Sharp 300 from what I remember, but then the Ultra Sharp 1200 feels more coarse than the Sharpal 1200.
Atoma I think is the standout among all diamond plates due to their design. The diamonds are all adhered in small raised clusters in an even pattern across the surface. Leaves a really nice and even scratch pattern. You can get the full stone which costs the same as a DMT, or you can get the slightly cheaper replacement "blade" then just use your own backer which is what I did.
The DMT's I own and have owned (dia-sharp models) all suffered from grit contamination. As such, they do need a bit more break in to get rid of all the too large particles for the stated grit. Once broken in though they do work well. I particularly like the DMT coarse the most as it provides a nice toothy edge yet you can still do light reprofiling off it too. DMT offers the most grit variety out of all the mentioned brands.
