Non muslim can not accept that any angel appeared and dictated a holy book. They never talk it openly, and find faults with islam and offend muslims.
Islam is very good religion, if we do not pay much attention to some words of quran, like smack the neck etc, and islam does not differ very much from some of the then existing religions, like Christianity, Judaism.
For one thing I am sure that (prophet) Muhammad had a very good understanding of human psychology, quran has answers to all human weakness, that (prophet) Muhammad succeeded in establishing a system for many coming centuries.
May be for one thing he failed, he did not know in advance how his followers would behave after him, if they will not start an infighting.