I'm a Christian, and I respect the Quran. I am honestly not sure exactly what to believe about the Quran. My opinion is that Muhammad was a true prophet, called to teach the people of his land. I would like to have been able to meet him and judge for myself if he was a prophet. I expect that yes he was. I have not read the Quran all the way through, but what I have read seems good. I wish I could read it in Arabic.
There are a couple of things that make evaluating Muhammad and his message difficult for me. 1. I don't understand the culture of the people at his time. 2. I cannot say for sure how much of his message survived, as opposed to being changed and filtered by those who handed it down and recorded it.
I know that cultural context is an important tool to understanding religion. The Jewish scriptures can be very difficult to understand to a non-Jew like myself. They are written by Jews and meant to be read by Jews. Someone living in Jerusalem in the 600's BC would probably know exactly what Isaiah meant. I, on the other hand, often have to struggle and work and research to understand what they understood naturally. Also, the message a prophet is given is always tailored to the people he is sent to. He is sent to minister to the lambs of his flock so to speak. God would not reveal a code of health regarding the preparation and consumption of meat in a completely agrarian society for example. There is also the matter of only revealing as much as people are prepared for. God teaches us in baby steps. He gives a little bit, to test our faith. Then if we receive it well, he gives us more. This process continues constantly. The law of Moses for instance was a preparatory law which was meant to lead towards Jesus Christ and the law he taught. Does that mean Moses' law was wrong? No. So, I guess in the same way, maybe the "law of Muhammad" was a law suited perfectly for those it was given to.
There is one thing of which I have no doubt. Good Muslims and good Christians are serving and worshipping the same God. They just refer to him differently.