@Nydex Silicon-based Lifeform should not be allowed in this discussion :b
Seriously though, we've only seen so little of the capability of silicon-based lifeforms, it is not a coincidence that silicon is just below carbon on the periodic table, I think it is premature to deny its life/sentient/intelligent/conscious status. If our consciousness is meant to survive millions or billions of years, those carbons have to be replaced with silicons
Crap, you caught me!
For real though, I've thought about silicon-based lifeforms quite a lot, and have read a fair bit of research on that idea, and unfortunately right now it seems rather improbable, simply because life usually takes the path of least resistance.
The biggest challenge for silicon-based life seems to lie in its chemistry, particularly its interaction with oxygen. While carbon-based life metabolizes to produce gaseous carbon dioxide, which is easily expelled via multiple routes, silicon metabolizes into silicon dioxide - a solid substance like sand/quartz.
So this creates the difficult problem of how such a lifeform would excrete solid waste. It essentially risks being clogged by its own exhaust, so to speak. Additionally, silicon-to-silicon bonds are weaker than carbon bonds, making it much harder to form the stable, complex long-chain molecules that we have observed as seemingly being necessary for life to arise.
However, again, we don't have all the answers. In the future our understanding of the biochemistry surrounding silicon might completely change based on some new discovery in another planetary system or galaxy...
It's certainly not impossible for silicon-based life to exist, particularly in environments almost completely devoid of oxygen (and by extension - water), but based on our current understanding of the science around it, I'd say it's
unlikely.