I haven't contributed a lot here yet because I feel like I've been out of the loop for the last 2-3 years, but some things never change and down riggers are one of them.
Sliders are a good tool that, like said, have their special days. But I think there's a few little tricks that can make them better.
Some days when fish are scattered and out of temp., a slider just stuck randomly on your line will pick up a few. Because if you think about it, they're just running above by them selves ahead of the rest of the spread. But like jatc said, I especially like them on deep riggers. Then I'm usually running a short lead on my main line and a fixed slider just above and ahead and it becomes a part of the spread not separated from it.
Also a friend showed me how to run a MUP rig, which is a fixed slider with a mag spoon run just ahead of and above a regular size spoon of the same pattern, theory being it's an attractor for the main spoon. So I quite often run mags for sliders, too (well I did a couple years ago, hope to this year, too?).
Also, a little off topic, but if like me you only run 2 riggers, I'd stack a rod above the main rod/line and use a slider on the stacked line, to give me 3 lures per rigger. But things gotta be pretty slow to be doin' that