Dirty secret time: the El Dorado 15 has been "artificially" sweetened. But you know what? I still love the stuff and think it's flat-out exceptional. Not just at the price, but as a spirit in general.
I suppose we should ask this question first: why add sugar? The short answer, according to some people who know quite a bit, is that it makes rum taste better and is something of an acceptable historical tradition. However, a fairly vocal contingent of drinkers and distillers have voiced the opinion that like artificial colorant, it tarnishes the spirit and conditions us to like (and expect) a narrower taste profile.
In all fairness, I'll admit I have something of a sweet tooth. I'll also add the following: if what touches my tongue is tasty, I usually won't care if they added sugar, artificial colorant, the blood of third-world newborns, or distilled unicorn tears. Some might give a haughty laugh and say, "Perhaps you can't appreciate a fine spirit unless it tastes like Coca-Cola." In response, I'd shrug. I like what I like and won't be brow-beaten into saying otherwise. That's kind of my thing here.
I'd also point out that the world is big enough for un-messed-with spirits such as the line of very good products produced by Clement or J.M., which as agricole rums go are about as dry (i.e., unsweetened) as they come. Life needn't lock us into false dualities, and picking a side in the sugar wars is unnecessary. There's already room on the shelf for a multiplicity of rum styles.
So if you can accept the fact that the El Dorado 15 has been screwed with, I say this: it's very tasty, and all theoretical faults are forgiven. I'd go so far, actually, as to say that the qualities of the ED 15 are balanced around the presence of sugar. At first sip, the ED15 is as rich and as powerful as a member of the Walton family. There's supremely tannic wood, brooding waves of cherry tobacco, and the kind of chocolate where they slap the cacao percentage on the label. What I'm saying is that some sweetness here is extremely welcome when it comes to complimenting the smoldering intensity of everything else.
Lacking the sugar, my worry would be that this would just be too intense, too wood-heavy, and too tannic. If it were a mixed drink, something about it would feel incomplete and in need of something to "lift" those flavors. Here, as it is with so many quality cocktails, you do have to balance savory around sweet. I think El Dorado has hit a home run in how effectively they've accomplished that. Sugar is an ingredient: in certain recipes, too little can be as equally awful as too much.
I'll end with this thought: recently, I've been more and more surprised by the number of people I talk to that just aren't that enthusiastic about rum in general—even people who are regular drinkers of scotch or mezcal. Certainly they don't mirror my enthusiasm about the category. Probably the easy answer is they haven't ever had anything on the caliber of the ED 15. If you want to treat yourself for really not that much money (at least in comparison to scotch or mezcal), I'd say this is a sterling example of what rum has to offer. At least for someone who orders a sweet cocktail every now and again.