A nice feature of MileagePlus Shopping, which doesn't seem to have been mentioned here before, is that it has 0.5 miles/$ for all bookings on Expedia, including flights. This is in contrast to all click-through cashback sites that I'm aware of, which have a small flat reward for flight bookings on Expedia and other third-party booking sites (e.g. $1.25 for Expedia flights on Ebates). For high-priced flights the miles should easily be worth more.
It's not a lot, but it seems like a nice little extra for basically nothing. Do you think this would be enough to justify booking United flights through Expedia rather than directly on the United site?