I was amazed by these levels, though I do have some nits to pick.
The architecture was stunning. I was a bit surprised by comments about there being a lack of height differentials, as I thought the deck plane was highly varied and in the E1 style. Comments about this or that room being "bland" also surprised me, as I found this to have a high detail level, without going overboard. I'm not a detail junkie, in any case.
The authors, in my view, expertly captured the basic design philosophy of E1. As others have mentioned, this style is simple but is rarely put into practice. Basically, you need to have a lot of interconnected spaces, with hitscanners spaced out at intervals to walk into rooms unexpectedly, or to snipe at you from windows, ledges, etc. You also need to see goals from places where you can't immediately reach them, and so on. The authors did this better than anyone I can think of with the possible exception of Sean Birkel and Jan Van der Veken, so Double Impact is up there with Fava Beans, Dawn of The Dead and The Classic Episode, IMO.
Monster placement was excellent, and often agonizing, especially when Cacos, Lost Souls, Demons/Spectres and hitscanners were all mixed in. I must have killed myself with rockets at least six times because of that. The levels were tough, but not excessively so, and I played all the way through E1M7 on UV. Individual levels were tougher than the entire Fava Beans episode. ;D I didn't play E1M8 because I don't like Boss levels, and E1M9 will be saved for later, and I'll start the map with an IDFA cheat.
I thought the authors achieved great suspense and tension, rare in Doom levels, through occasional quiet moments as well as fantastic light and shadow. There were a lot of E1M2ish dark computer rooms, and I enjoyed every one, though sometimes the flashing lights were very annoying.
There were some fabulous traps, too, even in the first level, though those were just a hint of what was to come.
I would have liked more plasma cells. A BFG would've been nice, too.
The only flaws, relative to my way of looking at things, involved some of the levels being too obtuse or too much of a switch hunt, especially given their size. IIRC, in E1M2, one of my faves from this set, the Blue Key was behind a secret door. And in E1M3 I never found the Blue or Yellow key until I started clipping. Likewise, I clipped to the Blue Key in E1M7 and didn't bother with the third, Yellow I think, and I finished that level with only 50% kills. I'll watch demos to see how people got to those keys, since the path isn't obvious in Doom Builder. To be concise, I'm not into playing Keepaway with keys as a way of making a level more difficult. To me, it just becomes more frustrating, and I start to cheat, since I have no intention of wandering around aimlessly for 30 minutes looking for a key.
Bottom line, I think this is among the greatest E1-style mapsets. These are also probably the best-looking techbase-style levels I've seen. Great job, guys!