Not an NBC, or CBRN as the current trend seems to be heading into a new designation, guy by MOS but I am the appointed NBC NCO for my company. I'm a 42A (Human Resources Specialist, 75B for the old folks) in the Army thats been to the CBRN Defense course. As an appointed duty it isn't too bad. I went to a two week course to learn how to run the equipment and train others to do so, the various reporting formats, and how to assess threats and the impact of actual attacks. Good for some promotion points and a few good bullets on the NCOER (When I finally make SGT, CPL for now).
My unit does not rate any more than a single E-7 54B at Battalion level, however in my last unit, HHC to a brigade sized element as opposed to the stand-alone battalion I'm in now, we had three 54Bs. As an MOS it seemed to be a pretty crappy field to get into.
CBRN hasn't been too big of an issue lately and tends to be neglected/over looked in the unit's training plan (at least the units I have been in). As such, the 54Bs tend to be pigeon holed into the supply section who in turn make them into the bitches of the section, ie they get put on all the crappy details so as to spare the S4 NCOIC from being short handed. The army has real trouble retaining junior 54Bs largely due to this. They just don't like to put up with it. In fact, a year or two ago they promoted all eligible E4 54Bs automatically to E5 whether they had been boarded or to PLDC or not becuase they just couldn't fill the positions they needed to with the available pool of soldiers.
FWIW if you can put up with that, promotions come pretty quick compared to some of the other fields, almost as fast as combat arms MOS's. Don't know if I'd like to do it full time but like I said, its not bad as an appointed duty. This may not apply to the USMC, I'll ask my borther but may have to wait for a while for a response, he's in Fallujah right now and doesn't get to email home very often.