Ok, I've had both bikes, here's my experience.
The YZ(X) is a great bike, it can handle anything you throw at it. Go read any review on a YZ and this bike lives up to it.
But my down falls were this:
1.They cut the head to different spec so that it's easier to kickstart and mellow power, but it created a weird flat spot at 1/4 throttle. This is fixed by installing the YZ (mx version) head, or cutting the head to give a higher compression ratio. I tried switching the CDI between the YZX and the YZ and I didn't like it. The Cut head and YZX CDI is a killer combo for offroad.
2. The transmission is a wide ratio 5 speed. For me, I needed a usable 4th gear for GP/MX, but the 4th gear was so far from 3rd gear I had to be cranking to PROPERLY use the 4th gear.
3. The little things. They put in softer clutch springs which was awesome for lever feel, but it didn't have great load capacity, so I had to replace the clutches often becuase I was over using the clutch. (I went to stiffer springs and that fixed it). I threw in a different reed block to open up the power, make it a bit more snappy. I'm also pretty sure the radiators are made of cardboard because they bent first time I crashed. So I threw on some rad guards to keep them straight.
Once these things were done, the bike was great, I didn't need anything else from it. But, I looked at what I spent getting there, and was close to what the KTM costed. For a couple more $ I would get E-start, hydro clutch, anti-vibration counter balancer, true wide ratio 6 speed, and a bike I wouldn't have to spend any money on to race. SO... I bought one. Other than the price tag, I don't regret one thing about it. In my opinion, it's the best bike i've ever ridden stock or mod. Power delivery is so clean and smooth, Suspension is phenomenal (once you learn how to set it up), and I swear, KTM designs the bikes knowing your going to work on them (bolts are easily accessible and overall design is simple).