I raced a 525 xc last year for motocross and it was very durable, easy to work on, and had great power. I cut vents in the airbox, put on a steering dampener, rejeted it, changed oil regularly, and ran the hell out of it.
2004 DVX 400
-FST 500 big bore/stroker, cams, valvesprings, revbox, ported head, Yoshimura full exhaust, Edlebrock carb, custom airbox/batterybox and headlight eliminator, elka shocks, janssen swing arm, gibson a-arms and steering stem, LSR gusset kit, AC nerfbars and bumper.
2008 KTM 525
-Rath nerfs and bumper, KTM a-arms, Twin-Air filters, Hiper wheels with razrs, precision dampener, Flexx bars, bored carb, 13:1 CP piston, 806 cam, Weapon exhaust, Renthal chain and sprockets, quad-tech hood, FourWerx parts
-2009 sponsors-661, Oneal, Works Connection, CP Pistons, SMX Optics, Amsoil, Weapon MX, Twin Air, Renthal, ASV Inventions, Steve's Sports Center, Chuck's Welding and Repair