I stick with Avis out of inertia. (Holdover from the days when I cared about collecting British Airways Executive Club miles and they were -- maybe still are -- giving 500 BA miles per rental instead of the measly 35 or 50 you get with other FF programs.) Now I'm just used to their website and major airport locations.

Upgrades are spotty, the "Preferred Select" loyalty tier means nothing, there are no real rewards for repeat business, rates and fees and gas charges can be exorbitant, and IMHO and experience, whenever you have human contact with an Avis employee it's disastrous. I guess I could be lured away...