There are known to be some problems (usually of omission) but those based on a 'parish register' are usually fairly good (always check in the IGI where the information came from and ignore patron submitted material etc) - the parish reg transcripts are actually not direct from the parish reg! but from the 1910/11 transcript done for the Rolls office in order to make up for the late start of civil registration - this transcript was done on Island and thus Manx names tend to be correctly read (allowing for the numerous different spellings) but non-Manx names rather less well in my opinion. Mary and Marg are an easy confusion. Remember that some clergymen were less than punctilious in entering info into the registers and there are known gaps - see the book by Janet Narisiman et al for some comment on these - the IoM FHS journal has carried info on missing entries
I have an article (held over from this quarter) on the IGI and manx records written by a very experienced genealogist that is now slated to appear in August issue of IoM FHS journsl (so still time to join if you are not already a member)