We used them for a while, it was a basically the conductors office, now the conductor rides in the locomotive cab and the caboose has been replaced by the EOT (End Of Train device) which is the little rectangular yellow/orange box on the very last car of the train. The EOT sends information about the train to the locomotive, such as length, air pressure, etc. You still see one on a train every now and then depending on where you are, what railroad and so on. I saw a caboose on the end of a short 20 car local train the other day. They are also used as shoving platforms when a train is going in reverse and the engineer can't see all the way behind the train.
Most of the time now, you can only see a caboose on display or on a tourist railroad.