Yeah, it's one of those things where unless you know what you're doing, I would totally avoid it. Because sugar is so fermentable, it contributes alcohol and nothing else - no lovely unfermentable sugars, no lovely mouthfeel, just alcohol. As a result, beers made with a significant proportion of sugar can be ass-nasty, with an unpleasant taste that's often described as cidery. If you wanted to make a stronger beer, you'd be much better off adding more dried malt extract (DME) - you can make some great beer using just DME.
On the other hand, I'm about to make a big Belgian beer, which will end up at around 10% alcohol by volume. For beers that big, adding some sugar can be a good thing, precisely because it ferments out so completely. If you made a 10% beer using only malt, there would be a lot of residual sugars left, so the beer would taste cloyingly sweet. In those instances, using sugar can be a good thing - but even then, only in small amounts.