Using Fox, you can add about a half hour each way to the time spent getting a car. At the airport in Vegas, you need to take a five-ten minute shuffle bus ride to the rental car terminal. All the other agencies are there, except Fox. For Fox, you need to take a second shuttle bus another couple of miles. The ride itself is fairly short, the wait for the bus varies.
Since it is not connected to the Rental Terminal, you save three dollars a day in fees, and it's generally cheaper on the base rate as well.
If you are willing to forego an hour of your vacation to save some money, it can be worth it. The cars seem average.
If you are not going at prime time, I find using hotwire a week out gets me great prices. I also book thru
http://www.carrentals.com. The thing to remember is that rates change constantly, and you can cancel a reservation right up to the time you pick it up. I lock in a low rate but constantly check to see if I can beat it.