A common belief is that is the best way to see how good someone is. While this is fairly true, it is more of proof of the player's playing style.
If their k/d is well over 1 or in the 2 and above range, chances are they camp more than the average player and don't come out of hiding very often.
If their k/d hovers around 1, they are the average player who will hide for a bit, but spends most of their time running and looking for enemies.
A k/d under 1 indicates that the player probably runs around like a chicken with its head cut off and gets picked off by campers.
Average player: Mine is 1.02
Runner: My k/d is 0.74
Kill Death Ratio doesn't really prove how good you are, more so the player's playing style.
Summary (may require some mathematical understanding):
If 0 < k < 1, or log(k) < 0, then you are a n00b.
If k > 1, or log(k) > 0, then you are a good player.
(k is your Kill Death Ratio)
Duude! I'm l33t!
What's your Kill Death Ratio?
0.5 *attempted puppy face*