the consumption of diet coke etc is not directly going to cause you to gain weight from its calories but it does stimulate the delivery of insulin to break down the sugar. Unfortunately there is no sugar to break down and so your body has just shot more insulin in to you than you needed. This can cause low blood sugar (hypoglycemia) which in turn can cause people to feel an increased desire to eat.
There also appears to be a growing number who believe there may be a link between certain types of cancer and aspartame.
Yes, that is possible, even though I still doubt the increased insuline theory. It's not as if our metabolism has tastebuds. It doesn't think 'oh, there's something sweet, let's create some more insuline just in case'. Insuline production is, simplified, a response to consuming sugar, not a response to consuming something that tastes sweet. It doesn't work that way.
For me personally, the opposite is the case - if I crave something sweet, a diet soda will usually take care of that, without making me want to eat more.
And even if all of the above was true - it still means that consuming diet soda on it's own doesn't even have the ability to make you put weight on. If a person sticks within their calorie range, and doesn't add any additional food, the consumption of diet soda won't hinder weight loss at all.
As for cancer, yes - in mice. I'm not a mouse. And does it really matter if I die of cancer because of the chemicals in my sweetener, the chemicals in the air, in clothing, in building materials, in other foods or because I was out in the sun for too long?
Most diet sodas don't contain aspartame anymore anyway (at least not the ones I drink), but the sad fact is that something that is supposedly safe to use today, might be linked with some sort illness tomorrow.
I think I'll just keep things in moderation, and hope for the best.