The K&K passive systems are one of the less obtrusive in terms of external appearance, but you do still need to drill a hole in the end of your guitar for the output jack - not for the faint-hearted, especially if the guitar is valuable. I've used them a few times on my guitars, and the results are pretty good, although of course you get no "on-board" control of volume, tone etc. The lack of pre-amp wasn't a problem in terms of driving my AER acoustic amplifier.
With the K&K transducers, you have the option of installing them permanently with gel CA glue, or just with double sided tape. You won't get as much output if you tape them, but it does give you the option of removing them at some point in the future (although you'll still have the hole in the end of your guitar).
P.S. if you use the gel superglue, be sure to use gloves when installing them - you really don't want to end up with your hand glue to the inside of your guitar.
James Lister, luthier, Sheffield UK