There has been a lot of talk about this. The good the bad and the ugly.
I will give my own experience.
So far it has saved from

losing 100% of my data once.
losing 1 file once.

i think these statistics prove the usefulness of timemachine for me as a user at least in my 5 months of usage.

Recently, after that 5 months it actually told me that my backup disk is full and will overwrite data 5 months ago. This is not a problem.

My rough usage.

My timemachine disk space 80 Gb (its 1:1 with respect to my macbook's harddrive)

My total use of my macbook's harddrive is around 55-65 Gb / 80Gb of my disk

i plug in timemachine an average of once every 2-3 days

i say thats pretty impressive!