I've used a lot of these over the last 15 years or so. Silver-plated, copper windings, over a thicker than standard core. They work really well with most gypsy-jazz style guitars as long as they're lightly-built. Some, especially cheaper, models have apparently been put together by carpenters rather than luthiers, and Argentines don't stand a chance (although such guitars are best avoided, anyway, unless you insist on looking the part even though you sound awful).
They also work well with any other lightly-built (and, typically, smaller-bodied) acoustic. Not on dreadnoughts or jumbos, certainly not on carved-top jazz boxes, but interesting on 00s and 000s, small-bodied Gibsons, and so on.
They're less jangly than brass or bronze, with more individual note definition—a more "old-fashioned" tone than orange or brown strings. They're not that different to Silk & Steel, to be honest—a bit louder, a bit more individual, a bit more sustain-ey.
If you like Silk & Steels on a guitar, try a set of Argies for a change. (The various recently-available Argentine clones I've tried have been, to a set, worse than the real thing, but as it's a newish market, that may change.)
People think they're only for Gypsy Jazz, but the fact of the matter is that they were the default strings for all guitarists in France until the 70s or so. If you walked into a French music store and asked for a set of guitar strings, this was what you walked out with.
If trying them on anything other than a Selmer-style guitar, remember to get them with ball ends (à boule) as mandolin-style loop ends (à boucle) are also available. Use the heavier set on shorter scale lengths, the lighter on longer—and pay no attention to the gauges given (.011, etc.) as they're not calibrated to match American-style strings.
Oh, and they work just fine with old-school magnetic soundhole (or fingerboard-end) pickups.
(It's also worth noting that some guitars, for some jobs (blues springs to mind) sound just great with "electric" steel strings on 'em. I've used d'Addario Jazz/Rock sets on a few acoustics and been very happy with the tone. But I'm not a frenzied strummer looking for maximum jangle...)
