My F800GS is Cat D. Largely superficial damage, but the cost of BMW parts meant it was quickly worth them writing off. I had the stuff that mattered from a mechanical POV repaired by BMW in a dealership for just over a third of the payout of the bike. I came out of the deal about £2.5K better off, at the expense of a slightly more aesthetically tatty bike.
Some insurers aren't keen on insuring CAT Cs and you need more extensive checks performed before it can be put back on the road, but CAT Ds are a lot simpler. The second insurer I tried gave me a fair price for it, which was no difference to a non-written off one.
Key things to be aware of:
1. Check it over even more thoroughly than a normal 2nd hand bike when buying. If this isn't something you know how to do and it's from a private seller, pay someone impartial and competent to do it.
2. When it comes to selling the bike, you won't get as much for it as a non-written off equivalent. On the plus side, this works in your favour when buying. This is solely a plus point if you buy it and then run it into the ground rather than sell it on.
3. If it's written off again in future, irrespective of who is at fault, the pay out will be less as the bike's perceived value (see 2).