Stats were added along with Achievements in the 1.4 update. It can be accessed in the pause menu and is a very welcome addition.
I'll start of with the negatives. I don't know if they could have implemented this but all of the stats did not include previously completed tasks. For example all stats start out at 0 even on words where you could have been playing for 100+ hours. As a result this doesn't truly reflect how much time and effort you have put into the game.
Now the positives: The range of statistics is enormous. It measures how many times you craft, get or use each item. It also includes distance traveled but foot, cart, falling ect. and time played. This range of statistics allows a more competitive side and allows you to compete against your friends to see who really is dedicated to the game . (Yes i know i am constantly discussing minecraft with my friends).
I have also heard that in a future update of the game that (The Mighty) Notch will be planning to make your stats available to been seen online on minecraft.net. This will allow you to pit your commitment to the game against others and most importantly easily compare your stats to your friends.
The addition of Stats in Beta 1.4 wasn't something people really saw coming but was extremely successful and I feel that it partners Challenges in redefining minecraft.