The 22/23 is generally considered (by the high end training community) to be a reliable gun for occassional shooters - like most cops. However, they tend to say that due to the slide being insufficient in mass to handle the .40 round, breakage and unreliability begin to occur much earlier than in other calibers. Thus, for those who plan to shoot high volumes, a 9mm Glock is superior because it is actually overbuilt for the caliber.
From what I have read, I would suggest that a .40 Glock has a maximum average life span of under 30,000 rounds while a 9mm Glock has an average life span of around 100,000 rounds. Neither life span is bad. However, I also understand that the .40 will need preventative maintenance to include some parts replacement starting (possibly) at around 1,500 rounds. The 22/23 is also know to be inconsistent in reliability when using an attached light. As a result, Glock is redesigning the .40 recoil system for the 4th generation of pistols due out in a few months.
What I am saying is strictly a summary of what I have learned from people who's opinion I tend to value. I realize some may have had different experiences with a single gun. I do not claim to be a definitive source.