I think the truest thing to say is that having them may make weight loss
appear to slow down. They almost certainly don't make
fat loss slow down - how could they? They're only 30 or so calories more than a shake.
But what they do seem to do in some people is make them retain water. Now, if you have a bar every day, from week to week, you're not going to notice them affecting your weight loss because you'll retain the same amount of water from week to week and what shows on the scales is fat loss.
If you have them every day for a couple of weeks, then have a week without them, you'll probably see a bigger loss on the scales than you did during the weeks you had a bar every day. That'll be because you lost the water you were retaining because you were having the bars. But of course people then think, "Ooh, I didn't have the bars - that must be why I lost more weight this week!" and then deprive themselves of them. Needlessly, I reckon - but that's just my humble opinion.
It is best not to have them in the first few weeks though, partly because of the munching setting off the munchies thing, partly because the chewy bars have more carbs in them (in the form of a fibre called malitol - which is why some people find having a bar every day wards off constipation
). The extra carbs may (in a small number of people) cause problems with staying in ketosis. However, the crispy bars have around the same number of carbs in them as shakes and soups so if you're one of those unfortunate sensitive folks, you can still have peanut, cranberry and mint bars.
Edited to add: It's easy in those weeks when you have smaller losses to blame the fact you had bars. But the thing is, we just don't lose weight consistently from one week to the next. In theory we should - and if we were machines, we would. But all sorts of things upset water balance, from being ill to being stressed to being hormonal, etc. The most dangerous thing you can do to your head on Cambridge is to expect that you'll lose the same amount of weight every week. You won't. There will be weeks where you lose 6 pounds, followed by weeks where you 'only' lose 2. The trick is to keep in mind your average weight loss over a number of weeks.