I admit that I don't always look at my computer when riding offroad although since it's a FlightDeck, I'll use it for the gear display. I find it most useful when I'm on those epic offroad rides and I'm following some sort of map. I use the trip distance and avg speeds to estimate time and distances.
I've always had a computer for offroad riding. I wish CatEye still made the
CC-8000 ATC.
That was the toughest computer I ever had but I've also managed to
launch it into the bushes in the past. Luckily I was able to retrieve it. My FlightDeck has also been through quite a few crashes and is still going strong however. For an offroad computer, you might want to look at the CatEye Enduro line. I think they're up to the Enduro 8 now.