Running a unit of Spanish Jinetes behind the enemy lines in MTW and decimating an entire unit in a couple volleys
Every time I tried that my Jinetes just got scared & ran away without doing anything useful at all
Consequently I could never understand why everyone loves them...

Not much of a user of Peltasts & other javeliners generally.
They are a pain in the ass if you don't have any counter units & thats what Iphicrates' reforms were supposedly about; faster, lighter, counter-peltast hoplites.

I do like using the slingers though :)

High velocity, light projectiles beat heavy, slow ones. (1 unit faster is better than 1 unit heavier because the velocity is squared, this is why we use flechette guns now not spear launchers)
Slings can do the whip-crack sonic boom on the release & apparently there are Roman helmets with sling stone size holes through both sides...