Muffin,
They work.
However, your best practice, re: smoothing the action, is to put a number of rounds (some suggest 1000) through the gun before slicking-it-up.
Also, you will achieve a better result if you combine new springs with some judicious stoning/smoothing of the appropriate surfaces in the gun -the rebound slide and it's adjacent frame surfaces, for instance. You don't want to remove metal, so much, as you just want to "smooth" these surface. Miculek's Trigger Job DVD gives a good tutorage of the process.
Benefits are a smoother, crisper action. Cautions include: too light on your rebound spring can = poor trigger reset and in combo with the wrong main spring, iffy ignition.
I only perform these mods on "broke-in" guns and the effort is worth it for range use. You probably will not notice the differences in an adrenaline-fueled shooting scenario.
Mike