Hotel rooms start at 10,000 miles per night and go up from there, with some premium properties listing for more than 50,000. Cars also start at 10,000 miles per day and vary by class of vehicle.
There are no blackout dates.
While this is a "first" for the airline industry, it's worth noting that Hotels.com has long offered a simplified rewards program where staying at any listed hotel gives one credit per night towards a free stay at any other listed hotel (with free nights costing ten credits), which can be a better value over time if you stay in mid-range hotels for business and then want to pick out luxury accommodations for special personal trips.
If you're a United frequent flyer, this new program is a pretty good benefit, especially if you're earning lots of bonus miles with a United credit card, elite bonuses, etc. If you do most of your flying with another airline, you're probably better off staying where you are.