I wondered if anyone here was familiar with Slovio.
Are the claims made for it valid?

As an esperantisto, I find Slovio interesting, even appealing. However, I also find it to have the same problems as Esperanto.

• There already is an international language. For the world at large it's English. For the Slavs it's Russian. For forty years the Slavs, Balts, and a number of other peoples learned Russian in considerable numbers. For the Slavs that was ridiculously easy, about like a Brazilian learning Spanish. Russian has far too much momentum to be supplanted by another language, especially one that is so similar.
• Esperanto itself has established its stronghold in this exact region. It's the only place where you can walk down the street and have a chance of bumping into someone who speaks la internacian lingvon. Esperanto is easier to learn, probably even for Slavs, for whom its simple Western European wordstock is not at all unfamiliar because of borrowings in their own languages. Despite the Universala Esperanto Asocio being headquartered in Rotterdam, Eastern Europe is Esperantujo. There are still huge Esperanto conferences and journals there, where almost all of my remaining Esperanto pen-pals live. Slovio is faced with a triple momentum problem: Russian, English and Esperanto.
• The creators of Slovio had the same problem as Doktoro Esperanto, insufficient study of phonetics. Sounds like H and SHCH are not universal in the Slavic languages. The former will be a challenge to everyone but the Czechs, Slovaks and Ukrainians, and the latter will frustrate everyone except Russians and Poles. Perhaps H is intended to be prounced as the Cyrillic X, as in Romanian.
• I'm not familiar with the Baltic languages, buy I suspect that generalizing from Slavic to Balto-Slavic may be pushing too hard. I am not sure that the Lithuanians and Latvians will find Slovio any easier than German.
• I have a hunch that Slovio has the same problem as Interlingua, or Latine sine flexiones. It may be easier to read than to write.
• In addition to the Balts, there's a political problem with other citizens of the former Soviet bloc. The Armenians, Moldovans, Azeris, Georgians, Hungarians, Estonians, Tajiks, Turkmen, Uzbeks, Kazakhs, Kyrghyz and others that I probably left off that list do not speak Balto-Slavic languages, and except for the first two they don't even speak Indo-European languages. Only the throngs of people in those countries who already speak Russian will have an easy time with Slovio, and they don't need it because they already speak Russian.

Esperanto was a more well-crafted language. I can't tell whether Slovio has the word-building facility that makes Esperanto's vocabulary so easy to master. That and the streamlined grammar are the key to Esperanto's success, not its comfortable familiarity, which does not explain its popularity in non-Indo-European Hungary, Japan and Finland. But that success reached its peak in the idealistic 1920s and Esperanto has been in decline every since. Financier George Soros is probably the last human being to have been taught the language from birth.

Slovio is trying to catch a curve that is rapidly attenuating. People all over the world are studying English and now Mandarin, or simply waiting patiently for computer translation.

Thanks for a thorough answer, Fraggle.
I wonder how far you can tell whether a Pole or a Croat (for example) would be likely to understand me if I addressed him/her in Slovio. The claims made on the Slovio website for its immediate intelligibility seem strong.

I think he'd be more likely to understand your writing than your speech. Pronunciation varies more between linguistic groups than we outsiders realize. In Czech, the accent always falls on the first syllable; in Polish it's on the penultimate; in Russian someone puts the word a dartboard and the accent goes on the syllable hit by the first dart--until it is inflected and then it goes back on the dartboard. Palatalization is absolutely rampant in Polish, nearly so in Russian, vestigial in Czech, and almost non-existent in Bulgarian. G occurs only in foreign words in Czech (and I believe also Ukrainian), replaced by H, a sound other Slavs mistake for KH.

Again I use the Iberian languages as a parallel. Most speakers of Catalan, Spanish and Portuguese can puzzle their way to the general meaning of each other's written language, but the cadence and phonemes of the spoken language can be baffling. Spanish holds the position of Russian, most Portuguese and Brazilians encounter it so often that they can come close to understanding it. But the sounds of Portuguese, with its nasalized vowels and other peculiar echoes of French, are confusing to a Spanish speaker. This is like a Russian being able to talk to a Slovene but not being able to understand his reply.

Thanks again, Fraggle.
What you say in regard to the Iberians is spot on -- so I am going to trust what you say about the Slavs!

i understand it

I feel that languages, cultures, and perspectives are intertwined. Thinking in another language changes the way one thinks. Because people natuarally hold affinity for their mother tongues, they generally appreciate foreigners who make the effort to learn their language. Taking the time to learn a traditional language pays much greater dividends than linguistic shortcuts, synthetics, and facsimiles.