They are equally useful and about as well developed.
Comparing Esperanto with Klingon is comparing apples and oranges. No, Klingon is not that developed as Esperanto is. Klingon is for a bunch of enthusiastic Trekkies who wan't to live their universe. Esperanto has a 136 year old history with a bunch of literature, music and tradition behind it.
No other constructed language has that.
Nobody uses it. You’ll be the only one.
What for a pile of crap of an opinion. Most languages can be described as "nobody" uses it. When you are outside of the top 10 languages in the world, then any language is a harder way to use it. You could move to the country or region where it's then spoken or maybe find the community in your city which speaks such a language.
Esperanto can be also found like that. There are people who speak it and also there are regular events where people can meet up.
Every book is not "accessible", when it's not even opened and willfully ignored of existing.
There is:
There are languages to which it's less accessible, but from the bigger ones, it's quiet accessible.
But if people don't open their eyes they don't see the forest in which they are standing.
Esperanto is eurocentric, because it's international. Because romance languages where made by colonialism of the roman empire. The argument goes of "equality". Thinking the other way around would be that asiatic languages colonized the world, then Esperanto would be based on asiatic languages.
Esperanto is a pragmatic language, not a "totally neutral" language. If you design a language to be "totally neutral" then parts would be distributed differently. How to chose which vocabulary of languages should be used often?
So using romance languages is a pragmatic solution to this. Through usage words can be added or fall out of use, all that is allowed in Esperanto and which can make the language out of colonialism in the future more egalitarian.
But it's ignorant to ignore Esperanto at all and morally vilifying it as "eurocentric therefore bad".
Oh I love it how people shit on Esperanto for it's euroncentrism. Argumentation by perfectionism, that's this. No one says that Esperanto could not evolve into a more egalitarian language, but feeling morally superior because its inventor chose to take from one cohesive language family (latin, romance languages) because the roman empire conquered the world a long time ago ... is silly and wrong.