The most fun is upgrading my lights,
It took me 10 months to get the parts for my nFlex WX1S LuxeonV 8AA to 2D Mag and the results were amazing. The regulator is programmable in case I want to push say... a W or X bin K2 (2007??) at an amp for a nice little upgrade. Some thermal epoxy and a soldering iron to radically change the lights output and beam characteristics. It's ultimate design is a throw monster with that large Mag head/reflector when the LuxV is replaced.
My original 2002 minimag went through a N bin, Q bin and R bin sandwich that varied between 416mA and 500mA of drive. NX-05 optics, Fraen LP optics and IMS 20mm reflector with custom bezel ring. Glass replaced the plastic stock lens and other various things have been done in the last several years before it retired to shelf queen.
The replacement for the LuxV Mag is a much easier (although more expen$ive) BAM! four K2 S bin Mag drop-in mod. HA-III bored Mag, UCL lens, McR20 reflectors and 8AA to 2D battery adapters with (I hope) Sanyo Eneloop NiMH Batteries. 400 lumens of fun and it will be easily modified with different reflectors, batteries, Mag bodies, K2 LEDs, tail switches, bezel rings and other things. Have a 2C Mag laying around waiting to be loaded with three CR123A lithium batteries for a quick and smaller 400 lumen blast.
My MillerMods L1P is waiting for a T bin (late 2006 or 2007?) K2 for 100 lumens of output in a single AA NiMH light. It always costs less when waiting up to a year for the ultimate bin so I am saving money. Sort of...
Add another vote for mods...