What is wrong with Splenda? Well to begin with it is an artificial sugar. It is made by chlorinating sugar. This process chemically changes the structure of sugar on a molecular level.
The reading I have done on it shows that there have been very few studies done on its safety for people. There have been no long term studies done to determine if there could eventually be health problems. No studies have been done on its safety for pregnant women or children.
There have been some studies done on rats and there are numerous problems for the rats (enlarged liver and kidneys, decreased red blood cell count, aborted pregnancies, diarrhea, to name a few that I read).
The makers of Splenda say it is not metabolized into our bodies but some studies show that about 25% of it is. They have no idea at this point what affect that could have on humans.
If you would like some further and more indepth reading: