Other than Esperanto, the only other language I can speak with some confidence is German, which I learnt at school to A-level. I also used to learn French (also at school) and Welsh (independently), but I am very rusty in both of these languages.
I started to learn Esperanto in 1994 (at the age of 13).
Because it was free to do so ... I had a book called 'Free Stuff for Kids' and one of the things I could do was 'learn another language', which involved writing to the Esperanto Association of Britain and asking to try out their Free Postal Course. (I was also generally curious about languages at that point anyway, so it was almost a no-brainer to write in and try it out.)
Mainly only when I meet other Esperanto speakers (not as often now as a few years or so ago), although I have also recently started translating subtitles in YouTube videos into Esperanto, which helps me practice my vocabulary.
I am not quite as fluent in Esperanto as my first language, but I can hold conversations about various topics (including some more specialist ones) reasonably well. I reckon my fluency now is similar to the fluency I had in German at A-level (which was 20 years ago): I don't think my spoken fluency in Esperanto has deteriorated nearly as much as my German has. (I have generally been better at written rather than spoken communication in any language.)
I think it has some value as a social international language: its adoption for official purposes has not really happened to date, but it can still help people from different countries to meet each other. (This assumes, of course, there are Esperanto speakers that one would actually like to speak with!)
I don't think that statement is true. Aside from the fact that only around 10% of the world's population can speak English (albeit more widely spread across the world than, say, Mandarin Chinese) and it happens to be very difficult to learn (even as a native speaker), there is a fair amount of cultural and historical baggage associated with English that could dissuade people from learning it. In any case, co-opting an unplanned (national or even regional) language intentionally as an international one is fraught with difficulties.
I work as a scientist and most published journal articles tend to be written in English - while that would suggest English as a de facto language of science, the language quality in articles can vary massively depending on where the authors are based. I have had to peer review several articles before publication, and sometimes just trying to understand what the authors are attempting to explain can be very difficult!
I would also add that I believe the implied claim in that statement that there can only be one international language is incorrect. Not only are other 'national' languages (e.g. Spanish, Portugese, French) used comparatively widely, but Esperanto is not the only language planned for international communication (although it does seem to have stuck around longest after its creation). Other created languages also exist (e.g. Ido, Interlingua), and someone might come up with something even better than Esperanto in the future. (That is not a reason not to learn Esperanto, though!)
The fact that it was designed as an international language, it has comparatively regular grammar, it can be learned quickly compared to other languages, it has a truly international culture that can come with it.
Not really, no ... I happen to speak Esperanto, but my personal identity is not defined by the languages I can speak.