Last company I flew for used to have the 421s and they performed but like Mike said, they are temperamental. You have to keep positive power on the props or it can cause problems for the gears if the slipstream is pushing them IIRC.
Im digging into the way back machine of 22 years ago flying 135 cargo; we had a small charter operator next door who would have some of our pilots on his payroll for their 414 and 421. Trying to remember the issues that were being dealt with. I do remember the 421 being in the hangar alot for general maintenace, as well as one of the 520s having to be replaced early for damage due to suspect mismangement.