I would say that most are well intentioned. But one can make judgment calls about someone's sincerity.
I'll give you an example. Senator Coburn is a guy I don't particularly like. I think he ought to be brought up on ethics charges for his part in covering up the Ensign scandal. However, any reasonable person who listened to his concerns during the health care debates could not doubt that he was sincere in his concerns. I disagreed with him, but he was sincere.
However, when you see Republicans filibustering their own ideas, even tax breaks for businesses, then you know they're not being sincere. When you see a record number of filibusters, you know they're not being sincere. When they ignore any proposal to address the economy, and instead vote for laws that are already on the books!