I put about 5 minutes on it, possibly got above 30MPH, and survived just fine. My immediate impression was "this thing will absolutely let your wrist write a check that your body can't cash."
The thing that bugged me, and kept me from writing a bad check, was that it's so squirly. I have very little dirtbike experience, maybe they're all like that, but the thing felt like it just wanted to fall. I never got around to riding the XR400 when we were out, guess I'll have to drag it out again and see how it feels. I rode Mike's Suzuki a little and don't recall having the same feeling, but I was so thrown by having my spine whacked when I went over the same little bumps that the 650 made unnoticeable.