Unfortunately, some of them WERE that awesome - but then, IME, the mostest awesomest ones were those least in need of having to hear it from others.
Still, no reason to wait until you can't tell them - tell them now, while they're still around to hear it. Most likely, they'll just shoot you with a blast from their awesome-cannon, making you feel a whole lot better about not having waited.
Those kinds of comments aren't for the dead person, who can't hear them. It's for the benefit of the grieving survivors, who can take some comfort in feeling that their loss is shared.
What siduri @3 said. It's about reaffirming community memory, of the person, and the things the person knew and remembered themselves. It is a loss to the community as a whole, and comments are for defining and confirming the (now-hollow) shape of that person in the community.
I remember thinking everyone said nice things at funerals and it almost made the kind things said pointless. Then I went to my great uncle's funeral. He was a total jerk in life. It was the least tearful and most quiet funeral I've ever been to.
A girl I used to babysit hung herself Tuesday night. She was a senior in high school, had been admitted to Amherst College in the Arts program, and was going to graduate early. There's already a memorial page for her, and it already has 900 'Likes'. I wish like hell she'd had some sense of this, and it had done some good for her.
Unfortunately, some of them WERE that awesome - but then, IME, the mostest awesomest ones were those least in need of having to hear it from others.
Still, no reason to wait until you can't tell them - tell them now, while they're still around to hear it. Most likely, they'll just shoot you with a blast from their awesome-cannon, making you feel a whole lot better about not having waited.