Do you mean the standard red/blue/yellow "Stakon" type terminals?
I use lots of them when wiring control panels and similar equipment using stranded MTW type wire.
I also use them (spade lugs) when attaching stranded THHN conductors to device terminals. Even if it isn't strictly required according to UL, it makes for a neater job, IMO.
The buttsplices are occasionally useful for automotive wiring, such as replacing a stereo or tying in an alarm system. The insulation-piercing tap connectors are generally pretty lousy, though.
The most important trick to using these terminals reliably is a proper crimping tool. The only tools that work consistently well are the full-cycle ratcheting type, with a crimp die that completely encloses the terminal while crimping. The standard plier-type crimpers can either damage the plastic insulation or undercrimp the terminal.
