I don't think that the amount lost every week should be the driving factor for you deciding on diet chef as its no quick fix. Some people lose loads really quickly, some people lose lots less.
it is a really convenient and healthy diet though, and because you aren't depriving yourself of real food you wll be more likely to stick with it, and then be more confident about portion sizes etc after that
my loss has been slow cos I've struggled at weekends, but I've still lost 12bs in 10 weeks other people on here of a similar starting weight have lost double what I have in that time.
Id say try it. As long as you stick to it it definitely will work.