I rented with
UT last May and had a back up reservation just in case. With UT, you are unable to link your Fastbreak number so you can't go directly to the garage. I tried everything including sending my adult son to the garage when we got there but they will NOT link a Fastbreak number ahead of time to a 3rd party reservation. The line for the budget desk was approximately 100 people long. I don't know how quickly it would have moved but I only waited about 15 minutes before I used my backup Avis (straight to garage) reservation instead and got a refund on my UT reservation. I was a bit worried that the refund wouldn't work since it was so close to reservation time but I did eventually get it. I have heard from others that the UT reservation is valid and there are no issues providing you don't mind waiting if there is a line.
My next trip is 5/30-6/8. Looking at $700 right now so I may have to resort to UT again. I am also looking at Turo but those initial fees are deceiving. It gives you an approximate total but when you go to book, they add on about $100 in fees. Still would be cheaper (I can get a car with good reviews for just under $500). Still contemplating.