I've flown 45's part 135 (about 500hrs a year on the aircraft), currently manage and fly one part 91 (approx. 250 hours a year), and do contracting in them as well. Even on the 500 hour a year bird we had about a 95%-97% dispatch reliability. I've always found the 45/40 to be very reliable. The only major issues I constantly see is in the APU, hydraulics, and AFCS systems.
Compared to the other Lear Jet types in our fleet our 45 was the most reliable plane that we operated.