I finally got around to playing this WAD again, and have just completed it, on UV MAX. Here's some feedback.
Overall, a wonderful megawad. Some favorite maps:
Sledge: The first difficult "slaughter-style" map. Ammo gets tight towards the middle of the map -- after picking up the first of the two left/right keys -- but otherwise the level flows well and the fights aren't too difficult.
A pet peeve, though: 100% items isn't possible without a (not very profitable) Archvile jump (see above).
666: I love the atmosphere here, particularly the starting outdoor area and the black pillar room in the dead center of the map. The *huge* outdoor areas to the side are kind of meh... there's so much room to maneuver that the enemy hordes aren't that threatening here.
The Pyramid of Death: I've heard people call this map "impossible," which is puzzling since it isn't even the hardest map in Speed of Doom... I'd put Map23, Map29, and maybe even Map26 over this one. To be fair, surviving the blue key area requires a bit of good luck, as it's easy for the Archviles to fail to cooperate, or to miss the Invulnerability during the Archvile jump (by getting hit with too many Archvile blasts at once, for instance). The rest of the map is not all that hard though: health and ammo are everywhere, and the monsters are positioned in a way that it's extremely easy to start them infighting at very little risk to the player.
The Shrine: A surprisingly tough map, given its small size. It's a war of attrition all the way through: Chaingunners, Revenants, the slime floor, etc whittle away your health and there isn't much around to pick up.
The Path to Hell: Brilliant! My second-favorite level of the WAD. The gameplay in all parts of the level is wonderfully balanced (except the very end, perhaps, which is rather easy... but the atmosphere of hell's gate opening, revealing its huge hordes, makes up for it), with lots of good cramped fights in both side key areas. The architecture here is also top-notch.
Poison Ivy II: This map is murder from pistol start, and no part of the level truly becomes safe until the very end. I want to particularly praise the placement of the Cyberdemon on the exit pillar as best in the WAD: from its perch it can snipe at you almost everywhere and keeps you constantly on your toes. You can choose to kill it early... but if you do, once the Archviles and Revenants pop in you'll wish you'd kept him around! A great, hard level requiring significant planning to survive.
Blessed Hellscape: "Twilight Massacre lite," but harder and more fun since ammo is scarcer (at least until the very end) and space a lot more cramped. A great map.
Hais Temple: The best architecture of the WAD. The initial fight in the center of the map is also tremendously fun: enemies are firing at you from every angle, and it's impossible to get at one group without exposing yourself to the others from across the courtyard. The outdoor fight on the way to the blue key, with the Cacodemons and Pain Elementals flying in as you're trying to deal with the Cyberdemon and his minor demon escorts, is also very well designed. The rest of the level is not without its flaws, though; the red key wing is too easy, the Cyberdemon guarding the switch in the blue key wing is rather gratuitous, and the Cyberdemons that teleport into the central yard at the end can be sniped from above with minimal risk.
Some low points:
Resurrection: I don't like playing Tyson-style, so this map is not for me, as I realized after punching out the four Archviles in a row at the very beginning of this map. Still... ugh. At least the level is short.
Hell's Honeycomb: The map itself is fine... but I have to call out the most gratuitous Cyberdemon placement of all time, in the Megasphere secret area. Shame!
Twilight Massacre: Don't get me wrong, I love a Resistance is Futile-style map as much as the next guy... but when the biggest danger in a map is catching a stray rocket due to lag, you are doing something wrong. Limiting the amount of the level visible at any one time by putting in more floor-to-ceiling cliffs would have solved the lag problem while hurting gameplay very little. A pity.
PS: I really hoped the green area at the start was a scale model of the entire level... but it doesn't seem so :-/
Darkness Without End: Meh. I hate gimmick Map30s, and this one is no better or worse than average.