I'm actually really happy when the shipper is Ontrac for me, they've consistently gotten me my stuff next day as opposed to UPS/FEDEX which do it 2 day.
Orders on Tuesday come Wednesday, as opposed to on Thursday as prime shipping would lead me to expects.
From what I've noticed : When I do Overnight it's fed-ex, when I do standard it's usually UPS. But Sometimes it's ONTRAC - and the stuff will arrive next day even if I don't pay for overnight.
I don't have problems with any carrier. Fedex delivers around noon, UPS Delivers from 3-6 in the afternoon, but they hit the expected delivery date.