The offer is only valid on flights marketed and operated by United, not other Star Alliance airlines. But this makes for a great value, especially if you can do a late summer vacation to Europe. Flights from San Francisco (SFO) and Los Angeles (LAX) to London and Paris in August are well over $1,000 round-trip, so redeeming 45,000 miles could end up getting you a value of 2-3 cents per mile.
Unlike previous United offers that were only valid for elite members or for people willing to travel on weekdays, this sale is bookable by anyone with United miles whenever there is Saver availability. Note that if you've already booked an award at the higher rate, you may be able to have the ticket re-priced by calling United reservations at 1-800-864-8331. It never hurts to ask!
TPG values United miles at 1.5 cents apiece, so even with the discount you may still get a better deal by paying cash. For example, if your paid airfare is less than $756 ($675 plus $81 in taxes), you may want to pay cash for your tickets, instead. Also, keep in mind that paid tickets earn miles, though award flights do not.
H/T: One Mile at a Time
