With only trace amounts of sugar and calories, I don't see how the max could contribute to weight gain.
There was a (tabloid-sensationalised) story a couple of years ago about increased risk of gum disease, when drinking lots of carbonated drinks, but I suspect that the well-informed journos forgot to differentiate between full sugar and sugar-free versions; there were reports that the sugar-free versions have high acid content, which did the harm. However, as with many media fads, this topic seemed to fade away quite quickly.
There could be other associations with affects on metabolism or absorption into the bloodstream, but I haven't seen any decent research on that either.