Having switched from a piezo to a magnetic pickup for stage volume/feedback reasons, I needed to switch from synthetic strings to steel core. I don't particularly like the sound or fell of all steel G and A strings, so these steel core, nylon wrapped strings are a really good compromise.
They have the same feel & guage as synthetic strings, with almost the same sound, but they work really well with magnetic pickups.
Sound-wise, they're not as mellow as synthetic or gut strings, but it's as close as you'll get and still be able to use a magnetic pickup.