They have an 1800hr TBO and the rear engine could get a little bit hot on the older ones, but this was mainly due to improper use or blocked cowlings. According to many owners they regularly get well over 2000hrs out of them. And the later T and P-models also had a bigger scoop for the rear engine, so it's only a problem on the oldest ones.
The US Skymaster club has a good forum.
Skymaster Owners And Pilots Site (SOAP) C336, C337, O-2