Tagatose is similar to Fructose, with which many are familiar. It is more sugar-like than any of the other sugar substitutes. Sucralose (Splenda) is the next closest thing. It is glucose with three hydroxyl groups replace with chlorines. The only possible downside I can think of with tagatose is that since it is similar in sweetness to sugar, you use a lot of it and since it is not absorbed completely in the intestine, there is the possiblity for osmotic diarrhea.
Scott the one you are thinking of is aspartame. It is the methyl ester of dipeptide, so it hydrolyzes to methanol in the stomache. Free aspartic acid is also formed, which isn't good for you either, but that is another story.
What else would you like to know?
Tagatose
Fructose