Just my experience (clearly others have not had it so good):

I just started flying frequently early to mid last year and have done so almost exclusively on UA, so i can't speak to much about other (with one exception). I have done roughly 8 international flights with UA, have never lost a bag. I was delayed a full day trying to come back home from SYD once (weather related, and eventually crew time expired), while certainly wasn't thrilled guess I was not surprised when you choose any airline with limited number of flights out of a certain city. They paid for hotel/food and flight resumed 24 hours later. I was delayed another time out of my home airport (again weather related but at connection airport), no real other airline option at the late hour i was flying (STL has limited options), at connecting airport, missed connection (this was known before i flew) again given hotel and food vouchers. They eventually upgraded me to first class on a different airline to get me to my overseas destination.

Which brings me to my one exception. I have flown international first class on both the new AA route to SYD from LAX and the UA route from SFO to SYD, I actually preferred the UA flight for comfort, service and food. AA had them beat on the bathroom as there were two of them for 8 people (and one was actually huge (up by cockpit) AA also beat them on the ability to turn the seat into a desk (rotate seat to window and have a nice desk with an ocean view

Had a few other experiences with DL and AA out of NY, every time (4 round trip totaled) have been delayed (usually a few hours), but i chalk that up to Airport. I do have to say i actually made it back from SYD to STL flying UA one time before a college made it back from NY to STL flying DL. Obviously something went majorly wrong there, but thought it was funny.

I am dedicated to UA for next several flights to SYD, Wife kids to SYD, a few family domestics and eventually a mileage spent to HI. hope i continue to receive the great service