On the whole the're pretty reliable things. They record speed and distance accurately once set up.
The only problems I've had are the date of max and min temperature reset itself back to the start of the year and now the daily trip meter only reads to 1.2 miles then resets. i guess it's a software glitch but it dosen't bother me at the moment.
I once had to replace the internal backup battery. Its a soldered in type but it's not to hard to do.
Also despite having a good seal on the lid and a couple of little bags of silica gel inside, the screen still mists up on a good day when the sun shines on it.
They are good instruments but I think the're too expensive, espically if you put on all the gear, roadbook lights and switches.
Still when it's all done, it's a real good job and definately worth it if you do any rally events.