I have had my Defender chipped. It's not a generic chip it's designed specifically for the TD5 engine by Bruce Davis in Sydney.
I've done 80,000 km so far an a lot of that has been hard driving - towing big boats and hard off road stuff.
Best thing I have ever done - engine is as strong as ever and gives just as much performance as when I put it in.
Again mine was designed specifically for my engine and I haven't heard any real issues from anyone else with the same chip.
(actually it's a completely re-map ECU - but same sort of thing in overiding the original controls).
Brett