I've had my Dillon HP-1s for about 6 months now.
I am finding them to be a mixed blessing. They do attenuate the sound w/o actually cutting it off, or 'clipping'. So, let's say you are standing next to a shooter who is emptying a standard cap Glock at a good rhythm. Your buddy is simultaneously talking to you. You do not miss out on every other word he says because the ciruitry 'shut off' at the blast. BUT, they do not provide the same level of blast reduction as straight muffs, and far less than plugs. IOW, in order to amplify ambient sounds, the gunfire must also pass through the cups louder than it otherwise would.
I've tried this several times during several different range sessions. The higher you turn up the volume, the louder the shots reach your ears. When turned completely off, they appear to act just like normal muffs. For example, I moved from my usual center bench at my outdoor club to one up against a sheet metal divider. The next discharge was noticeably uncomfortable compared to when I was in the open area. I turned them off, and completed the string unfazed.
The best of both worlds seems to be this. Install your normal set of earplugs, then set the muffs at max volume. In my guesstimation, it seems as though I retain 'normal' hearing, combined with the absolute deepest of suppression levels. When the spring pressure of the muffs starts to agitate me, or my sweaty ears need some air, I can remove them w/o risking my eardrums. FWIW, this is the only way I would consider the use of these devices in an indoor range. The reflection off of the walls would be far too uncomfortable(read dangerous) on the ears with it set at any sort of useful volume.