I've known a few people that bought ex-rental cars with low km's and had very good luck with them. They were good models and the folks saved a bucket of money upfront. Of course, it did hurt their resale, but if you drive the thing into the ground, who cares?
I think that many cars today are very well made and as long as you don't buy one that has had 100K of abuse, it's possible to milk years and years of good service out of a rental sell-off. I see 2007 Yaris' with 18K for about $5-6K off new price. That's an excellent car for someone on a tight budget that would like a late-model car. I would just double-check that Toyota will honour the warranty on something like that, but our firends bought an ex-rental Corolla and never had an issue.