There are two rental car companies at Lakeland Linder Airport: Hertz and U-Save.
The official website has information about both:
http://www.lakelandairport.com/
At most Hertz locations, the trick it to come up with a good rate code and/or coupon code. Employees of corporations can often use their Hertz corporate ID for personal travel.
U-Save is an unknown entity at this time. U-Save is a surprisingly large chain, but the Lakeland location is a franchise that just began in business in June of this year, coinciding with the beginning of DirectAir service. As with any franchise, if the local franchise owner is committed to great service and the location is well-managed, it could be a great choice. But, again, it's a new entity without a reputation or track record.
Anyone who willing to take a chance on DirectAir should probably also be willing to take a chance on U-Save.
For the time being, I'm sticking with American Air Lines into Orlando and National Car Rental when I get there. But that's just me.