Well... I won my first game because we messed up rules (and it was like 10 marines against 20 gaunts or something ridiculous like that), and that was a common theme for the first few years. When we started going competitively, my older brother's nids always killed me. I could normally beat or tie my little brother and his Eldar, however. Now, me and my older brother are pretty much even, winning and losing based more on what list we take than our skill. I play Blood Angels (with occasional White Scars allies) and my brother plays Nids (he's still figuring out the new book), but I don't win most of my games in 40k. Probably about half.
If you go down to the woods today, be wary