I have completed one before. a couple of times. I was never very good and probably couldn't do it again if I tried.
Though my...cousin in-law (that works right?) can do 3x3x3, 4x4x4, 5x5x5, but I think i stumped him with a 7x7x7. He could also do a 3x3x3 in...i think it was under 1 minute, 0:45 maybe.
They can be fun, if you know what you are doing