You can use piezo, transducers, built-in-mics - you'll never get the real sound of your instrument (steel string acoustic guitar). Use mic.
This mic is small, easy to use - no wonder it's used by many professional applications.
As it valid for any live situation with mic, it is recommended to have a sound engineer who cares about the feedbacks and helps in the soundcheck process.
My setups
Home recording: paired with an other mic, it gives a nice stereo sound - either it's mounted to the guitar or just placed beside the other mic.
Live, seated: mount to the instrument, plugged to the system.
Live, standing: oh, that's the best... it's an electret mic, so connect directly to the wireless sender on your belt, and you're free to move.