Aspartame is the most assiduously studied food additive ever developed, by far. If there were anything really wrong with it, it would be established beyond the level of rumor, where it seems to be now. I don't eat much food with aspartame in it, but that's more a matter of lifestyle than concern. It wouldn't bother me at all to ingest it. George Beinhorn's experience with a heavy soda intake gives some clues as to why sodas are not a good replacement for typical sportsdrinks, though they can be effective supplements to sportsdrinks.
Sodas also contain essentially no electrolytes.
A steady diet of sodas in an ultra can lead to electrolyte depletion if some other electrolyte source is not used along with the soda. George mentioned becoming hypothermic in his experience. It sounds to me like he was experiencing low blood sugar, electrolyte depletion and drug effects from the caffeine all at the same time - not a pleasant combination. So, sodas are not good substitues for sports drinks. Some runners favor Mountain Dew which also is a good source of sugar and caffeine, and contains the flavoring brominated vegetable oil, which may act as a stimulant in some people.