This article is from the Esperanto FAQ, by Mike Urban firstname.lastname@example.org and Yves Bellefeuille email@example.com with numerous contributions by others.
For a native English speaker, we may estimate that Esperanto is about
five times as easy to learn as Spanish or French, ten times as easy to
learn as Russian, twenty times as easy to learn as Arabic or spoken
Chinese, and infinitely easier to learn than Japanese. Many people find
that they speak Esperanto better after a few months' study than a
language they learned at school for several years.
A knowledge of Esperanto makes it much easier to learn other foreign
languages, and there is some evidence that it is actually more efficient
to learn Esperanto first, before learning other languages, rather than
to study foreign languages directly. For example, one may become more
fluent in French by first studying Esperanto for 6 months and then
studying French for a year and a half, rather than studying French for
two continuous years. The reason may be that Esperanto's regular grammar
and word formation and flexible syntax makes it easier to understand
other languages' grammar and rules.