The subcontracted delivery folks are a huge variable. I've had them promise to have my bag to me by 3pm (I came in on a 9am arriving flight that time) and not get the bags until well after midnight. Their excuse always seems to be, "The driver scheduled for today never showed up for work."

Unless it's a real hassle, driving to the airport is generally a more sure way of getting your bag in a reasonable time. The delivery people in my experience have had no affiliation with United whatsoever, and calling UA doesn't do much to get the bag there any sooner.