Ok, so I tried 3 times to get through the first chapter and failed, largely because I did not know what to expect from it, and the first chapter's pace is painfully slow. It was only after someone told me what it was that I should expect that I pushed through, and I don't regret it now.
So at its core, Homestuck is a story about the end of one universe and the creation of another, and deals with things like fluid time, cause and effect paradoxes, and the nature of free will. It contains within it Epic conflict, fun music and animation, like-able characters tasked with nearly impossible deeds, and it is presented in the style that resonanates with Internet users of a certain age (most dialog is reaveled through IM clients) and that harkened back to old adventure games of yore. (Zork comes to mind.). It also explores the nature of relationships in 'outside the box' sorts of terms and generally has great character development.
And then, it proceeds to subvert them all.
All of that said, there's a lot of chaff to sort from the wheat, and the pacing bugs me a little bit. I haven't quiet gotten to where the comic is currently, but I think I'm going to let it run just a little bit more before picking it back up so that I'm not checking it every day obsessively and can get my fill of story in one nice hunk.