Karl, regarding your reference to the Wikipedia article, I have three comments/observations.
First, remember that Wikipedia is only as good as the last info that someone edited into a subject. As has been shown on many an occasion, many people who post there have a hidden agenda or bias. This article shows a bias, IMO. It may not be much and could certainly be argued, but the article certainly lumps all fleet types into one rather unattractive bunch. Do not forget that Wikipedia articles and/or information may be 100% accurate and unbiased, somewhat accurate and complete, or very misleading and biased. It all depends upon the integrity and viewpoint of the author.
Second, I have personally purchased three off-lease/ex-rental vehicles in my car buying life and have had the best of luck doing so. These three cars have the honor of being my most reliable and long-lived purchases outside of one new car I bought. This includes my current 1997 Bonneville, purchased off-lease with 31K on the clock. I have had no issues other than the expected maintenance items, plus the standard UIM and LIM gasket failures. And they were not until over 125K.
Third, from a personal perspective...I believe it is much more important to carefully inspect a car inside and out and have a trusted mechanic drive it for a considered professional opinion, than to trust generalizations about groups of used car catagories. And frankly, unless I knew the person offering up a used car for sale, I would have more feelings of concern about that person'* care and maintenance record than I would from a large company that spent regular money to maintain vehicles for a fleet.
It certainly helps to see a record of the maintenance on a vehicle and many fleet operators have those records that they will provide on a vehicle they are replacing.
OK, there you have my 2 cents, FWIW.
Mike: 1997 SE w/ 183K, aka White Horse
, Gutted Airbox w/K&N, drilled 180* t-stat, tint all around, clear corners w/SilverStars, "PONTIAC" light up tail, PEMs, NGK TR55s