I don't think this has been discussed before. If it has....my bad.
Anyway, last week I chose to use my monthly free rental coupon to rent NCAA '08 Football for XBOX 360. When I got it home I noticed the receipt said if I chose to keep the game past a certain date that my account would be charged $34.99. ONLY $34.99 for a game that is still selling for $59 new. Sure you might be able to get a better price on a used one from eBay, but we all know that can be a bit risky.
My only reservation about keeping the game was that it was in a generic blockbuster case. But all I had to do was tell them that I wanted to keep the game and they gave me the original display case off the shelf.
So as far as I can see there really is no downside to this. I got to try the game for free, the disc was in perfect shape, it had the original instruction booklet, and I even got the original case! Not bad for 35 bucks.