I just sold my WR450 (03 model). It was a great bike and I will buy the 07 model when it arrives. However, dont think that they are unbreakable.
1)The valves do wear out. I had to replace my ti inlet valves after 2 years (they were noticeably worn, although bike was still starting). Dont quote the YZ/Wr400 as an example of Yamahas having no valve issues. These bikes had stainless steel valves which are a lot tougher than the later models and their ti components.
2) My starter motor clutch wore out. Plus I had to install the 04 upgraded slipper drive gear. These were both expensive repairs.
3) Lighting regulator blew and needed replacement.
Bottom line is all these bikes require $$$$ and maintenance time thrown at them if you want them to run at the top of their performance game. Honda - Yamaha - KTM. Choice your colour and pay your money.
1) First 426's have Ti valves. Neither my 426 or 450 have required any shimming as yet after many miles. I sold my '99 WR400 to a friend and recently did another valve check after many, many miles. Three valves were just barely out (.001"). Not bad.
2) The '03 WR was the dog. Yamaha fixed the problem in '04 and after that the starter problem seems to have gone away.
3) Haven't heard of any widespread regulator problems. Sounds like a bit of a fluke.
Bottom line I've owned three WR4xx's and all have been extremely reliable.