Rum is still a little sweeter than most whiskey though, so (depending on the sprit it used) I'd say go light on the sugar when making mixed drinks with it.
While I'm a big fan of the Rum Which Dare Not Be Named, and drink that over ice*, I will say the El Dorado is a big bargain - I think the 12 year is like half the price of the Z- and is still good enough to be enjoyed on its own and great in mixed drinks. The 15 year offers no significant improvement, if I remember correctly. One note though; I believe that rums use a different standard for their aging - when the bottle says 15 years, it doesn't mean all of the rum inside has been aged that long, but that some of the rum has been aged that long. That's what I remember from a tour of a rum factory in Barbados a few years ago, anyway.
*Here's my personal recipe for a cocktail I call The Mid-Life Crisis; pour 3 oz of the 23-year old rum of your choosing over ice, mix with equal parts nostalgia and regret, and garnish with a sprig of mint. Enjoy!
EDIT: after doing some research, it would seem that some rums, including El Dorado, list the age of the youngest rum in the bottle, which is the same standard as whiskey. In other words, a bottle of 12 year El Dorado will contain a blend of rum not younger than 12 years old. Other rums use the "solera" method, which blends rum of different ages and which may use the age of the oldest rum in the blend. Think of it as the homeopathy of distillation. Zacapa uses this method, which may explain why (IMO) the quality has gone down as availability has expanded.