Of the 33 orders I've had with Amazon this year only 2 or 3 have been delayed and they were due to local weather. UPS simply could not get the routes done in time. An email from Amazon notified me of the issue every time and it was perfectly understandable.
Obviously with 33 orders this year and we're not even done with February I use them a LOT. I couldn't be happier with them, though I will admit Staples Advantage has Prime beat on a lot of office and cleaning stuff, and their next day delivery is solid.