I just watched that, and it actually wasn't bad. You have to be in the right mindset to appreciate it. It's a 2 hour long, somber, slice of life movie about a family (and in particular, about a guy and his nephew) coping with death. I thought it did a very good job of showing what people go through when the inevitable happens. I also felt like I could relate to Casey Affleck's character, because I was put in a similar position when my dad died.
If that type of movie sounds like it might appeal to you, give it another shot. I don't know if it was a grand masterpiece like everyone is saying, but I definitely don't feel like I wasted my time or anything.