Reservetech,
I've been doing two a days for the last three weeks and here's what I've noticed. It's been a nice change of pace, because I haven't done a calisthenic style workout in a while. All of the pull ups, cleans, burpees, and thrusters are helping me open doors to new found muscle. I'm also starting to trim a little more bodyfat off I went from around 13% to 11% (I use calipers). 5 weeks ago I was 227 and now I'm 237 (at 6'3"). That's 10lbs, plus lbs in fat I replaced with muscle. Granted, it's probably the extra workout in the day helping and the massive calories i've been consuming at the DFAC; but there are a lot of different movements that I believe are helping my strength and size increase across the board.
I was really skeptical of crossfit because soooo many guys that I've worked with this year have gotten into it and praise it as the end all to working out. A few of the other guys and I joked that they had all joined a cult and started drinking the kool-aid. I looked at it like a fad and avoided it all year until now.
I feel that crossfit borders overtraining; most guys here do it six days a week and run. Most of them have added a good deal of muscle, but they didn't have a lot to start with so I chalk it up to typical beginner gains. Anyone who doesn't lift properly (squats, deadlift, bench) and focuses only on isometric movements never really see gains, and a lot of the guys I've seen were like this. I'm also a little worried to the strain that it puts on your shoulders. We've been doing the workout of the day on crossfit.com and it seems like our shoulders are getting blasted quite often. If you are doing shoulders in the gym, cut back and let those babies heal.
If I were you, I would try it. Work it in for a few weeks as something different. Get your extra calories, sleep at least 8 hours, and drink at least a gallon of water a day. It never hurts to change up the pace from time to time. But my counterparts that have stuck to it all year long are starting to look like skinny triathletes. Just my opinion.