cmwdjw: Although I don't know your car-size needs, dates, or location, I will leave this recommendation anyway: have you considered bidding for the car on Priceline?
I'm not ordinarily a PCLN fan myself, but the level of service you will get with a free certificate is similar to what you would get with a PCLN rental. (In other words, you may be required to wait in the "main" line, and you will probably NOT receive a courtesy upgrade past the "standard" car.)
My main beef with car rental awards is that "free" doesn't always equate to 100% free, after taxes and surcharges are loaded in. Different awards exempt you from different taxes (I don't know about HHonors/National).
Check out biddingfortravel.com if you are interested. People post their winning bids there, so you'll get a feel for what might fly and what won't. I see people getting standard cars for $80-100/wk quite often.
If you have a near-unlimited pool of HHonors points (and I know a lot of people do), then you might as well take the free car. But if you would prefer to hoard the points for a more valuable use, maybe you can get yourself a cheap Priceline car instead...
(I'd hate to give away nearly half of an ALON for a $100-150 rental car, considering my limited supply of points.)