You have to be more careful with them. IIRC, the 16340 is lithium ion, whereas rechargable CR123 are a different chemical composition. Running two or more in series can cause problems with reverse current and exploding batteries. Finally, not all flashlughts that run on 123s will accept 16340 due to differences is current strength.

Candlepower forums has a bunch of good info on them, including reports of battery explosions. I use them occasionally, but only in single cell lights. I suggest you do some research, if you haven't, before assuming they're interchangeable.