Dude, I've never been a FAC. I've also never poked myself in the eye repeatedly with a sharp stick, yet still feel qualified to advise others on the relative merits of ocular-plant contact, or lack thereof. I base my opinions of FACs purely on observation over the past 9 years.
I have nothing against FACs. The majority are very capable officers. I just think that the company line which says FAC experience is key to understanding the MAGTF and essential to career development is a load of BS. If it were such a great deal, the Corps wouldn't have to involuntarily assign so many FAC billets. If you're into the grunt thing, have a ball. If not, take your B-billet ass to the FRS or P-cola and fly some more. You should probably get your ground ticket punched by the time you're getting looked at for LtCol. Until that point, it's all about MOS CREDIBILITY, i.e. are you a capable pilot and squadron officer?
There are a million B-billets. The further you get away from the conventional FAC and IP jobs, the riskier it gets from a career advancement perspective. I'm not a careerist, but all things being equal, you should keep that in mind while making a decision--50% of LtCol pay is a lot better that 50% of a major's come retirement. OSO (officer recruiter) is a big one. NROTC instructor is hard to get, but an option. Postgraduate school is good, but you have to take a "payback" tour, usually 3 yrs, meaning 2 yrs school+3 yrs payback=5 yrs out of the cockpit. The Corps also has Foreign Area Officers and Regional Area Officers, who become specialists in certain countries languages, etc. That's an interesting track for a B-billet, but way off the career turnpike. Security Forces is another option, if you dig the gun club aspect of the Corps. There's a bunch, you just have to know who to ask when the time comes.