285-290K miles on my two '08 3500s. Engines are OK, MPG not anywhere near as good as my old '05 2500 T1N (totaled in a crash, was still running great at 501K - R.I.P.) or my present '04 2500 T1N.
My biggest problems with the NCV3s have been constant issues with DPF/EGR, glow plugs, injectors, front suspension and steering, transmissions, heat and A/C, cracked wheels that won't hold air, and driveshafts.
My '04 with 220K still has original glow plugs, injectors, turbo, all front suspension and steering components, driveshaft, wheels, etc. - as did my '05 at 501K when it met its untimely demise. In the '05s last year of service (about 90K miles that year - from 410K to 501K), I spent only $3800 on maintenance and repair items. I can drop that on an NCV3 in one bad month - they're ALWAYS needing attention. ($$$)
My fleet will soon consist of all T1Ns with driver's side sliding doors. Without the worry of rust-out from road salt here in TN, I plan on running them until they literally fall apart... but that won't be anytime soon.