I have never done it personally, but there is a lot to like about CrossFit, as it seems to embrace most of the principles of a good exercise program:
* an emphasis on intensity, rather than just 'logging time'
* challenging strength training
* regular variation workouts to build a broader-based fitness
These are significant advantages over other exercise 'programs' that seem to take a much narrower approach to fitness.
However, that intensity is something you need to work up to gradually, and there is a risk that beginners may overdo things, becoming demotivated when it is too challenging for their (current) level of fitness, or in worst case scenario, risking injury. It wouldn't be my initial recommendation for someone coming straight off the couch, but for someone who has established a habit of regular exercise and is looking to take things to the next level, it definitely has its attractions.
Also, as with all exercise program, weight loss is mainly about watching your intake - 80% nutrition and 20% exercise is the common saying here. Get in shape with CrossFit? Definitely. Lose weight? It can help, but you won't get results unless to tackle the intake side of things as well.