Two main reasons why I consider it to be "great":
Firstly, it's just a plain good story. The way that Homer paints pictures with words is just magical. This sometimes doesn't come across very well in translation - I'm fortunate enough to be able to read Homer in the original Greek, and the sound and the rhythms of the original hexameter verse are just amazing.
Secondly, and for me, most importantly, Homer is quite literally one of the "cornerstones" of western civilization. The ancient Greeks regarded Homer as the "source of all wisdom" - much like many people today regard the Bible - and, whenever anyone in the Greek world wanted advice or guidance on some moral or ethical issue, they'd turn to Homer for inspiration. In the ancient world there were professional poets, called "rhapsodes", whose JOB it was to go around to towns and villages, and recite Homer - these people knew the "Iliad" and the "Odyssey" off by heart (some 25,000 lines of poetry!) and were regarded as extremely highly revered members of society.
Sorry - I'm lecturing. Ancient history is my "thing" and Homer is just for me, such a "cornerstone" of the whole origin of Western civilisation that I sometimes forget that most people probably don't share my love of his works.