I have K&Ks in three Martins and a Lowden, and they sound superb. If, as I am, you're playing solo or in a band with other plugged 'acoustic' instruments, K&K is a good way to go - very natural-sounding. If you're playing in a loud rock-type electric band, they are less successful due to the usual feedback issues that under-soundboard PUs suffer from.

K&Ks are passive (no f***ing battery in the guitar - yippee!) but benefit from a good matching-impedance outboard Pre-amp. I have an Orchid Muting DI (no volume/EQ controls) which I use when I trust the sound-man, and an Orchid Acoustic Guitar Pre-amp (volume, treble, middle and bass controls, and muting button) for when I don't. The latter device mounts on my mic stand. As you'd expect, K&K also produce a range of DI/Pre-amps, and there are others that do the job well - Redeye is one that springs to mind, Headway do one that has switchable input impedance and will match too.

The problem is, of course, that this kind of stuff is constantly developing, new improved gear coming along almost daily so, whatever one goes for, it becomes obsolescent almost immediately.