The KTM950 is a neat bike, but it's really pitched towards adventure touring and mixed long-distance all-surface riding, not dedicated dirt riding. If you're skilled and brave, you can ride it in serious dirt (e.g., the Dakar guys) but for most of us mortals, it's WAY too big, heavy, expensive, and fragile to tackle serious dirt. It really competes more with the BMW GS, Triumph Tigers, V-Stroms, etc. which are big/heavy street bikes that have been massaged a bit to handle fire roads and the like. The KTM is by far the most dirt capable, but it is still a very big/heavy bike that can cost you thousands in damage if you have even a minor dirt wreck.
If you just want a dirt playbike to fool around, it's hard to beat the tried-and-true DRZ400S. Or the new KLX250S is getting good press. Both are cheap.
If you want something a little more pavement-oriented, then the KLR650 is not a bad way to go. But even it is a little heavy for serious dirt.
If you are willing to trailer to the dirt, then a whole bunch of great bikes open up that are much better in the dirt. MX bikes, enduros, etc. But I personally love the flexibility of a dual-sport so I'm not confined to an area when I dirt ride.
KTM's are great bikes (I own one), but I'd never consider them terribly durable or reliable. They are quirky and have expensive maintenance. They're thoroughbreds and oriented towards racing. The Japanese bikes are lower maintenance and easier to live with. If you want one of these big dual-sports, don't overlook the Stroms. They're cheap, reliable, and can be equipped to handle the same mission as the other big dual-sports.
Back to the 950, while many have had great service, the bike has had a high incidence of weird problems, particularly with the cooling and oiling systems. If you have your heart set on this bike, keep in mind that there's a fuel-injected version coming out that should do away with the carbs which have been troublesome. Unfortunately, the FI version is delayed for a year in the US so we won't see it for another 16-months or so. The carb'ed bikes are going to really take a resale hit when the FI version appears.
Don't get me wrong, the KTM950 is one of the coolest bikes on the planet. But it's not a pure dirt bike and it not a bike to buy into lightly.
- Mark