I used Fox twice after I was rear ended and my car was totaled. Both weeks I picked up on Sunday and Monday evenings and there was no more than one other person in line. I was really worried because of all of the reviews saying the line is long and slow, so I can only assume time and day play a big part.
Definitely make sure you double check the size of vehicle they give you. For the first week I reserved a mid-size vehicle and was given a Hyundai Elantra hatchback. I didn't think about it until it was all over with and I was at home, this is a compact vehicle.
The second week I reserved a full size since I couldn't even fit my sons stroller in that Elantra. I was very excited when they gave me a 2018 Ford Fusion Hybrid, which I wished I had the whole first week too. However, the Fusion is a mid-size; it is not a full size. I enjoyed the vehicle for sure, but after returning it a part of me was still kind of bummed that both times I rented, I didn't actually get the size I had reserved.
Overall I didn't really have a hard time or any issues other than the vehicle size being off, but I definitely can say I only went with Fox because of the low prices.