What's your objective? If you want to record with the guitar, especially a quality instrument such as a Martin, you'll get ten times the tone with a good microphone than with an under-the-saddle pickup. However, if you want to play live with a loud band and not get enough feedback to blow your guitar into splinters, an under-the-saddle pickup such as a Fishman will do the job. It's another tone altogether and it can sound good with the proper playing style. Check out Craig Chaquico's albums--he does this all the time and it's a unique but very impressive sound.
If you do get one, make sure it's active not passive. And unless you really know what you're doing, pay to have it done as you don't want to carve up your Martin. It requires some drilling of the body as well as shaving some material off the bridge saddle.