All mine have been good (once fixed for those bought with known issues) for reliability.
I've had 2 breakdowns in the last 7yrs - crank sensor (every v6 owner should carry a spare

), and something going pop in an engine after (in Basil Fawlty voice) "a damn good spanking". I'd say this is not dissimilar to any other car I've had, probably better to be honest.
The key to V6 reliability, in my humble opinion and experience, is preventative maintenence. People on the Rover forum frequently call me all sorts of things, because I frequently recommend regular fluid changes, and even more regular fluid checks, to which my reply is that I am not posting up with car problems

The weakpoints on V6 are:
Crank sensor - carry a spare!
Oil cooler - probably poor coolant servicing
Breathers - naff oil (CASTROL!!) or short journeys - use quality oil and do annual breather cleans
Warped cam covers - a myth put about by mechanics who are either lazy, incompitent, stupid or all 3. Or "Vauxhall Trained Mechanics". Use genuine gaskets, ensure breathers are clear, and (listen, all Vauxhall Trained Mechanics!) USE A BLOODY TORQUE WRENCH!