while i am NOT the aforementioned chem expert, here's the nutshell version of the sweetness concept: your tastebuds have receptors on them that molecules of specific shapes fit into. sweet-sensing tastebuds have a pocket that's shaped like a glucose molecule, so when those glucoses drop into the receptor, the info is passed along your tongue nerves to your brain, where you process it as being sweet.
things that are physically shaped like sugar molecules will also fit into those receptors, some better than others, some "sticking" in there longer than others - leaving an impression of being more or less sweet than other substances that trigger those same receptors. artificial sweetners are of course imperfect fits, and are shaped like things other than just glucose, so you end up with odd "aftertaste" patterns for most of them. the wikipedia article on xylitol claims that it has "vitually no aftertaste", which matches my experiences of the super yummy xylitol-based chewing gum i get at the asian market.
quick note from your friendly vet: remember that xylitol is TOXIC to dogs, so don't feed this to your pooch friends.
