I have a 2008 690 (650cc) Duke. It's had countless owners and god knowns which parts on it are actually original. It has about 70k km on the odometer (assuming it's the original one and it was always fitted and connected). I've had it for about 6 or 7 years now I think.
Things which have been a problem:
- Clutches are consumables. They seem to last about 20k km for me. They're not expensive or difficult to replace yourself. The feel on them is fantastic though so I'll happily replace the plates if that's the trade off. I don't want to go back to a bike with a cable clutch.
- The cam chain tensioner. I replaced the hydraulic one with a manual one a while back.
- The muffler. This is not a KTM fault, a previous owner replaced the OEM with a cheap aftermarket can and it's shit and constantly needs repacking and rivets replaced.
- The single front disc on the front is prone to "warping" where it builds up material in one spot and pulses at slow speed. I replaced it once and the new one started doing the same thing after a short time so now I just deal with it. The same thing happened on my previous bike with a single front disc.
- The fuel mapping has been played with, and in combination with the supermarket exhaust is not perfect. I've put a standard map on but it's still just ok.
Overall I'm pretty happy with it. I often look at upgrading but all I really want is the same thing but maybe a bit faster.