It is one measure of performance but it is not a perfect one. For several reasons:
a) Carrier will most likely prioritise outbound traffic over handset delivery notification
b) The handset processing the message add an extra latency to the delivery receipt path
c) When absent subscriber no handset delivery receipt is provided in most cases until the phone is connected to the network again
d) they can be easily faked by operators
In order to measure the success of our delivery we use a number of sources, of which DLRs are just one of many.