It's a funny thing about rum: It all is distilled from a fermented sugar cane derivative (fresh cane juice, molasses, or something in between), but sugar doesn't pass through the distillation process, so some producers add sugar back into the distilled rum to sweeten it up.
Recently on Facebook, people have posted lists of sugar levels in commercial brands of rum. One reportedly comes from the Finnish government and another from the Swedish government. I won't post them directly here but the lists are here on RefinedVices.com.
You should really take a look. They show commercial brands of rums with anything from 0 grams/liter of sugar added up to 46 grams per liter.
**Note - You should verify the content against the originals. The Swedish government's site is: http://www.systembolaget.se (thanks Magnus!) and the sugar content is listed for some individual products. I don't know about the other list so I can't verify its authenticity nor how current either of the lists/measurements are. Furthermore, I do not know if the rums we get in the US are the same as in other countries, even if the labels match. In other words, do not treat those lists as absolute truth.
But how much sugar is that really?
Allegedly, specific bottlings of rum by brands including Plantation, Bacardi, Zacapa, and Angostura contain between 17-22 grams of sugar per liter. This looks to be the average amount of sugar added by those brands that add sugar. Other brands are either much higher (40 g/l) or quite low (5-9 g/l).
So I wanted to know how much sugar is 20 grams and how sweet that makes a liter of liquid taste. So I measured it. My scale isn't the most accurate thing in the world, but here's how 20 grams came out:
Which comes out to a heaping half ounce, or probably a little under 3/4 ounces.
Heck, it's not uncommon for there to be that much sugar added to a single cocktail. And keep in mind that's the quantity for a full liter of rum, not 750 ml bottle.
I then added the sugar to 1 liter of water to see how sweet it was. In water, it was certainly noticeably sweet, but not soda-sweet.
For example, the driest soda I can think of is Q Tonic's grapefruit, orange, and lemon sodas. They have 13g sugar per 8 ounces, which by my calculation means 55 grams of sugar per liter; more than twice as much as is these rums.
I'm not saying that you want your rum as sweet as Coke, nor you necessarily want any sugar added to your rum at all. But for these rums having what sounds like a lot of sugar added to them, it's not really a ton.