SmartOS, as noted in another comment, is not a general purpose OS. It seems to have the most convincing story behind it for why you might choose it over Linux (but, it hasn't convinced me to do so). And, it has perhaps the largest organization behind it (Joyent).
Honestly, though, they all seem to be niche products or projects. There just isn't the kind of large community one would need to compete with Linux or even the BSDs. If it were a community that was growing, it would be less of an issue, but everyone involved seems to be old-school Solaris geeks who just want to keep working on what they know. Which is fine, and I hope it goes well for them. But, it doesn't look like the future (once again, I think SmartOS has the most compelling story for IllumOS having some small piece of the future).