Eh, I'm not sure I'd call him "serious". He's definitely pretty good on some civil liberty issues. Rather less so if you happen to be female, non-white, or gay. Even if that wasn't the case, his understanding of economics is sufficiently loony as to exclude him from serious consideration, at least in my view.
In any case, a belief in civil liberties shouldn't even be a discrete political position; it should be the bedrock on which all political positions are founded. When one candidate defines themselves as "pro-liberty" -- as opposed to all those other guys -- the game is already lost. Should they win power, they'll need to claim extraordinary powers in order to reinstate "liberty" -- undermining democratic institutions in the process, and thus the actual mechanisms that guarantee liberty.
In fact this is more or less exactly what happened with both Bush and Obama, and I'm pretty sure that the same thing would happen with Paul. His message is: "I will give you liberty" -- not "I will ensure that the institutional checks and balances which act as guarantors of liberty are maintained and reinforced". In fact he rarely shows anything other than contempt for those institutions. Therefore I do not believe he would be a friend of liberty.