Ah hell, thought I'd commented on the first 5 maps already. Time to rewrite a bunch of shit. For the record, I've played this megawad several times over.
Map01: My first PWAD map ever, and I won't forget the feeling it evoked when I saw that pentagram for the first time. It's hard not be biased, as it does a great job in starting a megawad - short, violent fun. The curved halls are plain, but acceptable. Drawbacks include a touchy 30-sec door (completely unnecessary) and a trio of useless demons at the exit. Not bad, not great.
Map02: Some nice visual touches save an otherwise fairly boring play. The empty crossroads, repetitious monster closets, and absurdly distant secret path really grind things down for me. Only the CG battle is memorable at all. That being said, the bridges are great from a scenic perspective and that solitary archway is incredibly memorable for being a one-off thing. Oh, and custom textures++
Map03: Take those keys away, and this is a bonafide Map01. Very much typical of Mustaine's style, what with the Map01 theme being used. The highlight of this map is easily the pair of traps early on - a stair-lift massacre coupled with a rising wall that I didn't even notice the first two times I played the map. Little things help it stand on its own feet, and it desperately needed it.
Map04: The real meat of the megawad starts here. Typical Jens-style map. Switch-triggers everywhere, creative use of rising/lowering floors, puzzles here 'n there, etc. Unlike his other maps I've played, this one never quite grew on me for one reason or another. Gameplay's one continuous grind, without a real memorable trap and the SG can be lame (getting the SSG's annoying, IIRC). The exit's touchy... not a favorite of mine. Fun note: I once chainsawed an HK to death on this map and I don't recommend it.
Map05: Ah, yes, Flaherty. I really like this map. It's a great imitation of Romero's theme with some more 'modern' (for the time) approach to progression and visuals. The ledge-based gameplay isn't nearly as annoying as it could be, thanks to the rad suits and some space on the ledges, the monster placement is great (save for those sergeants near the start), and the arachnotron battle early on is a stand-out for me. Nice stuff!
Map06: Theme-shift galore, but it's a fun map. I notice a lot of people SR'd to the blue key, which is something I always found amusing. The route isn't all that great, as there's some backtracking to be done, but there usually an assortment of new enemies to keep you busy regardless. The YK-area was unique enough, with the darkened-outdoor trick. Serviceable, entertaining little map to run through.
More to come.