Hi Squirrel,
I've done Slim in Six three different times (over the past three years), and have had very good results each time. I typically lose around 7-9 lbs. in the full six weeks. I am 5'7", currently around 153, but when I last did Slim in Six I was down to 143-144. Goal was 137, so I was getting pretty darn close. (Then I did nothing between Christmas and March, so here I am back again . . . *sigh*).
I have done a ton of beachbody workouts (Power 90; Power 90 Masters; Turbo Jam, Slim in Six). Each is good, though in very different ways. IMHO Slim in Six is the hands down winner if weight loss is the goal. You won't build your strength as much as you would with, e.g., Chalean Extreme, but the calorie burn is huge. Also, the lower body workout is fantastic. Slim in Six is the only BB program that I have done that quickly produced visible, significant toning of the hips/thighs/butt. If you read the BB site, promotional info, etc., you'll see that SiS is the program most directly focused on weight loss.
That said, it is a tough program, and the last 3 weeks involve about an hour a day, six days a week. There will be days when you want to throw a dumbell through the TV. If you are new to exercise or prefer something more "dancey" (I don't, but I know some do), I'd consider Turbo Jam. On the other hand, the good news is that BB just released a "new and improved" Slim in Six, which looks pretty cool. Debbie has cleaned up some of the irritating aspects of the original (hella awful music, miscounting, overwork of one side of the body), and introduced some new moves. I am currently doing a round of P90 to get back in the swing, but my plan after that is to do a round of the "new" Slim in Six to lose the last 7-10 lbs. (I feel justified buying it, since my SiS is so old that it is actually on videotape!), and then on to P90x for sculpting purposes.
I have never done the six day "express" diet, though I know many who have. I think that most people lose 5 or 6 lbs., but I am always skeptical of anything called "express" when it comes to weight loss. I am 43, have thyroid issues, and am a *notoriously* slow loser. Maybe at 25 I could have lost 10 lbs. in a week, but those days are behind me.
Good luck with your choice. I'd recommend browsing the BB message boards for a few hours before deciding. You can generally find folks who match up with your height/weight/familiarity with exercise, and get a better idea of what you can expect. (Don't get me wrong -- stay on this site too!). It is also a really friendly community. Can be a little aggressively friendly, since many of the posters are "coaches" (e.g. independent sales reps) but it has been my experience that the advice and support offered is very genuine.
Again, good luck!