One running joke in Erik Tenkar's running of the Castle of the Mad Archmage is "Is it a 5% slope?" The dungeon features some sloping passages, the kind that take you down levels of the dungeon. The use seems predicated on the old AD&D/white box D&D assumption that sloping passages are undetectable, and can lead players to accidentally go deeper than they think they are.
We run them as obvious to detect, something I agree with, and we cheerfully use them to go deeper and deeper into the dungeon.
So, sloping passages are a staple of dungeons going to back to the earliest days.
"Passage south "D" is a slanting corridor which will take them at least one level deeper, and if the slope is gentle enough even dwarves won't recognize it." - The Underworld & Wilderness Adventures, p. 5.
Is it hard to detect them?
AD&D and white-box D&D say yes - you need a dwarf, with their special sense for underground direction and depth, to detect them.
"Detect grade or slope in passage, upwards or downwards 75% probability (d4, score 1-3)" - Players Handbook, p. 16
I'd personally say no, it's pretty easy to notice a significant slope.
A wheelchair ramp maximum slope is 1:12, so it drops 1 foot vertically every 12 feet horizontally. A more slight 1 foot drop every 20 feet (1:20, or 5%) is still pretty significant. Less steep than an ADA-compliant wheelchair ramp, but not terribly so.
But if you've ever walked down a slightly sloped street or sidewalk, or stepped from a flat grade to a sloping grade, it's pretty easy to notice. Setting a treadmill to a 5% grade is really noticeble (more so down, than up, although it's a rare treadmill that'll let you do that.) You have to not be paying attention to really miss a sloping passage dropping down or going up. And if you're not sure, any liquid or a marble will prove it to you. One joke in my Dungeon Fantasy game is Gort, the dwarf, demonstrating the secrets of AD&D-ish dwarven special abilities. Sloping passage detector? He always has a clay marble and/or a plumb line. It's even easier to detect them if you're pushing or dragging something - it'll suddenly feel heavier.
So the old "so slight you don't notice" thing is tough, because you need a downslope so slight that the passage goes on for a very, very long distance. Take a very slight drop, say a 1 foot drop every 100 feet. To drop from a series of 10' tall rooms to a series 15' below it (giving a 5' thick ceiling) takes 1500 feet of tunnel.
If they're detectable, how do you use them?
You can use them like any stairs, really.
Pretty much, you need to give people a reason to go deeper. Harder challenges, more loot, or whatever. A lure is better than a trick, here.
You can also get cute and give out enough treasure that the PCs need wagons or sledges or wheelbarrows to move it. Valuable statues are a good choice. Carrying is a lot rougher than, say, taking a sloped passage. If that statue on level 1 actually fits on the pedestal on level 3, you'll get players eager to find and take sloping passages.
Using sloping passages makes it possible for rolling juggernauts, boulder traps, daleks, and slithering snakes to get around the dungeon. It's wheelchair accessible, so wheeled or rolling critters will get around better.
You can also get cute and make them chute-like: slick, low-friction, etc. This makes them more of an obstacle or decision point, since going down safely might be difficult, and getting back up is no longer a trivial act of turning around and going back.
Mostly, they act in practice as just a different way to change levels.
They do make for some mapping issues, of course, because it's not always clear when the "level change" occurs. That alone can make them a bit of fun - you know what to put down on paper, but it makes it a bit of a worry if you get all meta with "which level are we on, and what does that mean for monster level?"
I think sloping passages make a terrible trick, but they make perfect sense within a dungeon. They allow for some different monsters and gives an alternative to stairs, pits, and ladders for getting around.