I've ridden with lots of guys and also seen lots of motors at shops with valve problems. However, my '05 CRF250R never had a single issue and I also know a handful of guys I've ridden with CRF250's and 450's that have never had problems. My bike was ridden hard and raced regularly in motocross and Hare Scramble events. Same with my buddy and his CRF450. Matter of fact, the last Hare Scramble I raced, I was waiting fo my line to go. It was a dead start, so everybody was practicing starting their bikes and taking off. I put mine in second gear, pulled the clutch in and kicked once and the bike started. I did that probably 10 times in a row before we started and these guys next to me couldn't believe it. They were asking me about my clutch setup and carb settings. Bike was 100% stock.

Needless to say I got the holeshot on that race too.
From what I've observed, 99% of the guys having valve problems were easy to see why. They're usually the guys who don't maintain and service their bikes the way they're supposed to. These are high performance racing engines and they need to be treated that way. I changed my engine and trans oil after every weekend (probably 8-10 hours at the very most). Air filter was cleaned and re-oiled after
every ride. I also pulled the valve cover and checked the valve clearances as recommended. Another big thing I noticed with the guys having problems is that they ran cheap gas, usually 87 octane because it's cheaper.

These are all things that you could get away with on an XR250, but not with a CRF. Keep the fluids fresh, run good gas, do routine maintenance, and these motors will last for a long time.
I talked to one kid that had a brand new 2006 CRF250R and the only way he could start it was by pushing it down a hill. He didn't even know where the oil drain plug was, nor what kind of oil to use in it. He was also asking me things like how to adjust the pilot screw and how to check valve clearance. Just goes to show that he never did anything to his new bike and expected it to keep running like it was new.