Splenda is Scary Stuff
Here's some homework for you: If you know a chemist (preferably an organic chemist, but anyone with a semester or two or organic chemistry will do), show them the structure of sucralose. Ask them if they would eat it. If they don't recognize it as Splenda immediately, I bet they'll give you a disgusted look and say you're crazy. Then tell them what it is. If they are pouring those yellow packages into their lattes, they'll stop. Every single chemist I've run this experiment on has the same reaction: "that stuff can't be good for you." Try it yourself, let me know what responses you get.
Explaining why this molecule is likely not good for you can be a bit complicated, but I'll give it a go. It's those chlorine (Cl) atoms that are so troublesome. First off, the structural comparisons mentioned above are terrible chemical metaphors. There is nothing (aside from it being the same element) at all in common between sucralose and table salt, as FDA Chemist George Pauli would like you to believe. DDT is slightly closer in chemical space to sucralose, but it's still pretty different, and its just irresponsible to try to scare people by making that comparison.
Understanding how those might react in the human body is way beyond the scope of this post, but let me make a much more realistic chemical analogy for you. The potentially harmful portion of Splenda is most like methyl chloride (CH3–Cl). Not in that sucralose is a colorless flammable gas (you knew that), but that methyl chloride and sucralose are both good candidates for substitution reactions. This can lead to the modification of all sorts of molecules in your body, which is hardly ever a good thing.
The short of it is that long-term studies of the effects of eating this miracle sugar have not been done. Splenda's FAQs might suggest otherwise, but I'm skeptical. If the FDA thinks this is chemically similar to NaCl, then we've got some pretty big problems on our hands. Not only does the FDA ultimately decide what is safe for us to eat, but most people don't know enough chemistry to even think twice about their decisions. That's a failure of our schools and our government.