I spent 2 years making Splenda in Georgia and Alabama. That stuff is strong in its original form; 600x stronger than sugar. The packets are cut 100x with fillers.
I've never noticed any ill effects from it, but I have heard that there is the potential for it to degrade to two separate compounds that have some level of toxicity.
I do know the FDA spent 11 years investigating it before approving it.
After it was approved, Johnson and Johnson built a 400 million dollar plant to produce it an hour north of Mobile, Alabama. BIG business. That stuff is everywhere now.
The main difference between Splenda and aspartame is you can cook with splenda, nutrasweet is not heat stable. They both taste like crap to me, though. Diet sodas are awful.
As far as smoking goes, I thought most people gained weight instead of lost it. I think Grady said he played much worse for awhile after he couldn't smoke while playing, but Diliberto has commented that certain players quit smoking and started playing better.