Schaller Oyster S/P

PROS: Great little compromise between a microphone and an under-saddle pick-up. The under-saddle pick-ups fitted as standard to many electro-acoustic guitars tend to give a 'brassy' sort of sound. Because you can place this pick-up anywhere on the sound-board you can choose more treble or more bass as you like (by proximity of fix to bass or treble strings), plus you are picking up directly the acoustic properties of the instrument from the soundboard rather than the bridge, so you get a much better tone, The 'blu-tak' adhesive is plenty sufficient for a seated gig and doesn't stain the surface. The sensitivity of the pick-up and the tone are great (for a transducer).
CONS: The wire is thin and not robustly attached to the pick-up, so don't start doing Peter Townshend impressions! I carry a spare! The pick up casing is also prone to loosening and rattling (a smear of blu-tak can cure this). You will probably need a pre-amp (the signal is quite weak without it). A simple foot pedal type 'booster' for about £25 that doesn't 'colour' the sound is best.
OVERALL: Does the job, and surprisingly good sound quality for the money.