I used to be a hardcore primaries-only kind of guy. I simply didn't want to deal with the hassles of rechargeables. But then my friend convinced me to use an AW RCR123 in my Surefire ML-1x...and the difference just blew me away. The same is true with a few stock Surefires and LED Drop-ins for the older lights.
You get brighter light in the LEDs that take them, plus you always have the option of going back to primaries for emergency situations. For lights that I use every day, I use rechargeables. For instance, I have two RCR123s for my ML-1x. One is always charged and ready for instant swapping out when the other is done. I always have 6 primaries in a spares carrier in the car and a box of primaries around somewhere...so if there was a long period of time that I'm unable to recharge the battery (pretty unlikely), I could fall back on primaries.
I honestly never thought I'd be using rechargeables in my Surefires...but the protected Lithium Ion batteries have come a long way in terms of usability and knowing that I don't need to spend so much every year on primaries makes this hobby a little bit easier to manage. Granted, I still use ONLY primaries in my M3 and M6...but I hardly use those lights anyway, so it's not a big deal.