It's my favorite series. Has been since the first time I read Gardens of the Moon. Abercrombie's First Law a close second though.
It's hard to explain why I love it. From the first chapter it pulled me in and doesn't let me go until I'm done (excepting the first few chapters of House of Chains ;)). The scope of the story is one thing. How events hundreds of thousands of years ago shape the events in the books, the chronological jumps, how seemingly unrelated incidents suddenly become entwined. Erikson is a master at making every puzzle piece fall into place; even if you're confused about something as you read it, a few chapters or a book later it will make sense. The little throwbacks to events in previous books, etc. Besides that, his worldbuilding is phenomenal and his characters are unforgettable.