I'm with the Golf Junkie.
I've had very few failures with bolt on C/B's, and lots with plug-ins, except JBD's favorite - QOs (JBD, I love SqD stuff) The bolt on failures have been poorly installed, either loose or cross-threaded so as to seem as if they're torqued down, even though the connection is still loose.
It hasn't been limited to field installed breakers.
Try taking a torque driver to a factory assembled panelboard sometime. (well, everytime, actually) You might by pleased, or maybe disgusted. Don't trust 'em.
I think Caselec's got the best option, if you can hold out for the leadtime go for the factory assembled version>
