I think utopias are desirable and that it is instructive to think about them and to try to envision what a better world would be like. That way, it can be possible to correct some of the problems in the existing world. It may be impossible to achieve perfection, but that doesn't mean we don't need goals and ideals to aim for. Even if we fall short, as we will, if we get closer to a better society, that is an important achievement.
Utopias are rebuke as well to Pangloss's philosophy that everything is for the best in the best of all possible worlds. El Dorado shows, if we haven't figured it out already, that Pangloss's Europe is anything but the best of all possible worlds.
El Dorado, in contrast, is a place I would gladly choose to live. There is no religious strife, want, greed, or hoarding, and everyone is treated as an equal. People live in peace. It is a place where a person can develop their talents without fear of starving or being killed.
However, I see two problems with El Dorado. First, it is isolated from the rest of the world, which is how it survives, but while the society takes care of its own, it is the ultimate "gated" (or mountain-protected, as it were) community, turning its back on the rest of the world. Second, as Candide's reaction shows, people need education and time to "detox" from competitive cultures that encourage the desire to be superior and have more than others. Candide and Cacambo are seduced by the urge to take the jewels and gold that have no value in El Dorado back to Europe so that they can be rich. They do so. However, that plan does not work out so well.