I don't trust him 100% either, but on most issues he is on our side, and I think this article outlines the main arguments.
He might have thought it was a good speech (if so I disagree) but since then he has been attacking Obama's policies mercilessly.
Read what K. has to say in this article about Obama's arguments against Ryan:
"major demagoguery"
"Critics are describing Ryan's Medicare reform as privatization, a deliberately loaded term designed to instantly discredit the idea."
"The final charge cutting taxes for the rich is the most scurrilous. That would be the same as calling the Ronald Reagan-Bill Bradley 1986 tax reform "cutting taxes for the rich." In fact, it was designed for revenue neutrality. It cut rates and for everyone by eliminating loopholes"
But he also argues that even though the GOP is right on the issues, Obama is a coward who waited until Ryan proposed a plan to reform and then uses that plan as ammuntion for demagoguery. And we need the US voters to see through the leftist sophistry they have fallen for many times in the past.