It's not entirely pure strength, thats more powerlifting or weightlift or even strongman. Crossfit's thing is functional strength. It takes components from the majority of fitness regimes, combines them in a way to complement your lifestyle. Think "Pure" strength combined with muscular endurance. That's what I train for. Also, as Myles and Dexter mentioned, weight gain is directly proportional with your eating habits. I've been doing it for 3 years and have never weighed more than 71Kg. I can squat twice my bodyweight, deadlift 2.5 times. I'm down to 65Kg now and still have that same strength.
