Aspartame tastes OK and is a known carcinogen. I hear splenda is less toxic, but it does not taste like sugar. More of a fucked up acidic chemical flavor that isn't even sweet. Then again, I know some people who say it tastes just like the real thing and put it in everything so maybe my sense of taste is just weird.

This has some useful info:

[link to www.holisticmed.com]