tl;dr Influenced by? Yes. Mainly/truly/only based on? Nope.
Islam did not miraculously pop into existence (ok, to some extent it may have), but it was heavily influenced by pre-existing religions. This includes the name of Allah (Muhammad's Father's name includes Abd Allah, servant of god), which was the name of one of the deities worshipped in Mecca, and which is etymologically related to Elohim (one of the names of the old testament god) according to the OED.
The moon symbol also predates Islam, though its use on flags has only been traced back to the 12th century, and it was also associated with the Mithras cult, a variant of which is also known for its similarities to Christianity (link to a site that suggests Christianity is basically Mithraidism with a new name).
Importantly, the moon was not a symbol of early Islam. Thus, the claim that Islam is based mainly on a moon worshipping cult becomes difficult to defend, unless one would assume that the most holy symbol of the religion was somehow kept secret from everybody else for several hundred years.
While it will be difficult to deny that Islam was influenced by the prevailing thoughts and religions of the time, it is wrong to claim that "in reality", Islam is a pagan religion. The reality of religions are based the beliefs of the adherents, and if Muslims believe that theirs is fundamentally an Abrahamic religion, then that's their reality. Using the history of religion to make claims about the "validity" is just trolling.
Two more links for your reading pleasure (ok, only the second is a pleasant read):