I think I'm about 2/3 through 1999 mode.
spoiler (click to show/hide)
invading the pinkerton factory right now
It's pretty easy and I don't even play many FPSes. Only died once so far. I was right, as usual. Pick up the Carbine and Hand Cannon for almost 1 shot headshots w/ 50% crit perk, use possession on any machines, max out your health and shields, and camp out in well guarded corners. Ammo and health are all over the place. Turrets are easy to hide from and juke once everything else is dead. I've got about 4000 funds and not much to do with them, no upgrades interest me and death or checkpoints are pretty much a non-issue.
The reason I came down on this mode is because of Ken Levine's endless blustering in interviews and official blog posts. Too bad if you're an insecure fanboy that has to personally attack me for giving my opinion on an overhyped difficulty level. The game itself is a pretty bland shooter imo. The rail combat is kinda neat but that's about all it has going for it. The plot is barely keeping my interest and the pinkerton fetch quests have been dragging on for too long.
Now if you want a recently re-released shooter that might kick your ass, get Duke Nukem 3D and set it to Damn I'm Good. You run out of ammo almost instantly, you die in 3-4 hits and every enemy respawns within 5 seconds unless you gib them. Pretty similar to Doom on Nightmare. Or just give Vanquish God Hard a try.. that is completely ridiculous.
Once again:
"I appreciate their attempt with 1999 mode but people are going to find ways to break it easily. You can't turn a power fantasy sandbox game into a serious, balanced, and difficult action game by restricting reloads and buffing enemy stats. Western devs have typically been miles behind in hardcore skill based gameplay."