You can spend a lot on a PDC, but - it will fail someday. It's a matter of when, not if. Like the man said: "I think you will find a number of divers who don't "trust" computers. Sure they are reliable. BUT, they are electronic instruments, with a $10 battery, often in saltwater. Law of averages says you are tempting fate if you place all your trust in a computer." They're high tech, fancy, electronic tools made to be competitively priced - not for NASA moon shots. And some of those have failed, too.
I dive with two computers, same brand, same readings, if one fails (as happened on last trip) the other one has almost the same info. Other instruments I dive with every time as I wouldn't trust a PDC...
>> SPG. Much better track record than the hose-less gizmos.
>> Magnetic compass. Nice, simple, dependable.
>> $15 watch from Wal-mart - water resistant to 100 meters, stopwatch: I want to know what time it is in case the skipper gave us a clock deadline, and I want to manage my stops myself - with PDCs tracking the time, depth, loads.