He's a credible outlier, a valuable voice. But we need to understand that he is an outlier. I do not have the capability to judge the conclusions of one professor at the Univeristy of Alabama - Huntsvile, so typically I look for confirmation either from other schools such as Stanford, MIT, Yale etc or from other institutions such as NASA, Pew, Woodshole, etc -- but nobody agrees with him.
Spencer certainly is proof against the theory that the AGW theory is only due to governments' funding only AGW proponents. He is employed by a public insitution and paid with government funds.
Nothing I've read of his suggests that CO2 does not cause warming. If I understand him correctly, he differs from the consensus only in believeing that clouds opperate to counter warming rather than to accelerate it as most others believe. But I do not understand what he believes has caused the warming in the first place.
Understanding global warming requires expertise far beyond what one person can have. Spencer is contributing his work to the discussion, but he cannot creditably offer alternative explanations because he does not have the expertise. This is the importance of the IPCC assessment and other universities' full-department conclusions -- they look at hundreds of studies (all listed in their references) and took the imput of experts in many different fields to issue a robust report.
As much as I appreciate Spencer's work, I have to note that he is just one guy pretty much working alone. His parter at UAH,, Christy, does not seem as adament in Spencer's conclusions and his department and university do not seem to be as publicly in agreement as those at other universities. He is not just a guy in the "denier" community, his hypotheses are at odds with other scientists who don't believe in AGW. He has generated a layman following, but among climitology scientists he is alone. Worth reading, but unsubstanciated.