Can't give you any numbers on the operating costs. I flew/fly a 421C with RAM engines, winglets and speedbrakes. It flies very stable, has a very comfy cabin and is less noisy that other a/c in its class. The engines ( the RAM ones) use close to no oil, run very smooth and can be flown lean of peak without problems. Cruises faster than a King Air 90 at high levels. Can be flown with takeoff power from takeoff to landing ( wouldn't do that though...bit costly), climbs like a rocket if you firewall everything. Spacious cockpit for a piston twin. Ours has air conditioning, very useful during summertime. No seperate heater to fail as heat comes from the turbos...
Landing gear has some issues, and if you reduce the MAP too much during the descent the turbos won't maintain cabin pressurization. Doesn't take crosswind too easy.
My guess is reliability depends on the installed avionics, proper maintenance, previous owners...
Hope this helps.