I've been doing CrossFit for close to two years now. I wouldn't recommend it for everyone ... less and less so for anyone over 40 simply because of the number of injuries I've witnessed. While CF has been an overall positive experience that has transformed my body, I am not fanatical about the program like so many CrossFitters are, and actually have a lot to criticize.
With that said, if you go three times a week and give it your all each time, you will more than likely be in the best shape of your life after 6-12 months ... that is if you don't get injured. I don't know your age, but if you're over the age of 40 I would highly advise you to modify the workouts, particularly any of the wod's(workout of the day) that include high rep olympic lifts. And for the sake of your longevity, do not let any of the knucklehead twenty/thirty-something coaches tell you how much you should be lifting. Forget the clock and concentrate on good form, always good form ... not something CrossFitters are known for.