Do dimmable lights actually need a special bulb and a special switch, or can you just turn any lamp down low? You need both ends to agree with each other. A dimmer switch paired with a bulb that isn't rated for it will buzz, flicker, or just refuse to go below about 80 percent, which is not dimming, that's just a light with a cold.
The difference between a good dimmable setup and an annoying one usually isn't the switch, it's everything feeding into it. Cheap LED bulbs marked dimmable often dim in visible steps, so you go from fine to dim to suddenly dark with nothing smooth in between, and some hum faintly at low settings, which you won't notice until it's the only sound in the room at eleven at night. A proper setup uses bulbs and dimmers from the same compatibility list, which sounds fussy until you've lived with the alternative. For fixtures themselves, look at where the wire meets the fixture and how the dimmer module is housed, since a lot of plug in dimmer cords are just a thin inline box with the cheapest components they could fit, and those are the ones that fail first. A well made lamp or fixture has a dimmer that turns smoothly through its whole range, with no dead zone at either end and no flicker when you pause partway down.
For sizing and placement, think about what the light needs to do at full brightness before you think about how nice it looks dimmed, because a fixture that's underpowered at 100 percent will look sad and gray at 50, not moody. A dimmable floor lamp in a reading corner needs to actually light a book at full output, then you dim it for the evening version of the same corner. In a dining area, the common mistake is hanging a pendant sized for drama rather than for the table, so it throws light in a tight circle and leaves the ends of the table in shadow no matter what setting you choose. Measure the table or the seating area first, then pick the fixture, then worry about the dimmer.
Dimmable lighting suits rooms that do more than one job across a day, a living room that's a workspace at ten and a place to watch something at nine, a bedroom, a dining room that hosts both homework and dinner. It matters less in a hallway or a laundry room, places that only ever need to be on or off.
The good ones have a dimmer that you forget is even a dimmer, because turning the knob or sliding the switch feels like one continuous motion, not a dial fighting you in fits and starts.