I started the switchover to using 18650 batteries when I got my Jetbeam BC40 as it either takes 4xCR123 or 2x18650. I'm very happy with them, excellent runtime compared to a AA-batteried light of equivalent brightness and much cheaper in the long-run than stocking up on CR123s. Some CR123-based lights do not accept 18650s, just make sure that your light of choice does. Downsides to 18650 being that replacement batteries certainly are not as easy to come across as the super-standard AA/AAA/etc type, and that there will be a slightly painful initial investment in the charger (if you get a good one, which you should).
One important point to make: If you decide to go with 18650 batteries, do not cheap-out on either the batteries or the charger. Some barebones, super-cheap 18650 batteries do not come with built-in protection circuits which are mandatory for lithium batteries, unless you like lithium fires in your pocket. It's also important to get a good charger to prevent over-charging (again with the fires). I use AW IC batteries, charged in a PILA IBC charger. Candlepower forums is a good place to research the various well-known, reliable offerings of both batteries and chargers, and known-good combinations thereof.