Maltitol is a sugar alcohol (polyol) derived from corn, which is often used as a sweetener in diabetic food products because it is metabolised by the body less quickly than sucrose, and does not, therefore affect blood sugars to the same degree. And, depending on who you believe, it doesn't feed and promote yeast overgrowth either. Neither does it have much of an unpleasant chemical after-taste like many artificial sweeteners.
The nasty side-effects of aspartame are well known, so when I'm looking for an occasional treat (let's face it, we all need a bit of comfort food from time to time), I tend to avoid anything containing it. Much of Boots' (the Chemists) range of diabetic sweets, chocolate and biscuits contain both sweeteners, but their dark chocolate mint discs and shortbread fingers, just use maltitol, and the chocolate discs are dairy-free to boot! So I've availed myself of a couple of packets to get me through that 'time of the month'. And they're really not bad. Diabetic chocolate is never going to be the same as normal chocolate (and I still crave a bar of Dairy Milk from time to time), but it's not far off and the shortbread fingers are a revelation: buttery and delicious, with no perceptible difference from 'normal' shortbread. Great!
My nutritional therapist assures me that products containing maltitol are fine occasionally - possibly not for someone just starting out on the diet (it's a really good idea to deal with that initial sugar craving and in time lose your sweet tooth), but for me - maintaining - it's okay. Of course, excessive consumption is not recommended; for a start large amounts of maltitol can laxative affect!