Interslavic is another similar project. They don't like Slovio too much: http://steen.free.fr/interslavic/index.html http://steen.free.fr/interslavic/faq.html#slovio
As a speaker of Croatian (southern Slavic) and Ukrainian (western Slavic), both Slavio and InterSlavic are bad from the Croatian POW, but InterSlavic is definitely a better choice in either case. Many word forms in InterSlavic just seem like strangely archaic versions of Croatian, but might pass as "just strange" for Ukrainian.
As a speaker of Polish (western Slavic) reasonably familiar with Ukrainian I find InterSlavic and madzuslovanski (or whatever it's spelled) much better than this Slavio.