It's a drink.
And the manufacturers claim that when you drink it, it could cause you to burn up to 200-odd calories without lifting a finger.
The idea is that there are ingredients in it that have a high metabolic cost to process. It's the same idea as Weight Watchers focused on in their new Pro Points scheme. Everything you eat has a metabolic cost. Some things are simpler/easier to convert into energy and building blocks than others; some things require more energy spent by your systems to perform that conversion than others. And there's been a (fairly limited and not statistically significant) study carried out by Leeds Metropolitan University which seems to show that resting energy expenditure could well be appreciably elevated for a period of time due to drinking Aspire.
I'm not the biggest fan of cranberry, but the taste is actually quite pleasant. This is mostly because it's not overly sweet. There's a hint of the "seltzer" to it and I guess that's down to the added vitamins etc. As a soft drink goes, I think it's one I'd think about buying to keep the variety up (especially while I'm Not Drinking). And with talk of there being a new flavour launched in March, it's a real candidate to add into the variety rotation of drinkies to stave off the taste boredom.
If there really is an effect, it might be a nice little helping hand to general calorie expenditure, but even if not it's quite a nice drink.