I used Avis booked through BA a handful of times for roadtrips in the US and have found it to be pretty good - inclusive insurance good enough for my own circumstances, free second driver and good prices. Last time, I took it on a 10,500-mile journey.
There is always the usual upsell attempt at an Avis desk, but on the whole a generally ok experience, or maybe I've just been very lucky. I know a famous FT uncle uses BA/Avis too and is very happy with this approach.