I switched most of my rentals to National this year. I have not had any trouble using the free days, but you don't get some of the same "options" for the luxury etc that you get with the reward points.

The "Free days" was not the driving factor on my switch. (Pricing for quality received was)

Can't speak to Avis, but they keep sending me emails about a special going on right now where if you rent a couple of times you get a free day. (Not planning to rent anything except WITH the free days I already have for the rest of the year before the special expires so I haven't really reviewed that deal in detail, I am sure there are details on the Avis forum)

Honestly, over the years I have rented from the all....the difference is only "different words" but IMHO there's no real difference anymore. I am using mainly National because I have a good contract rate there. I am pretty much "price loyal" now. Service with the car rental companies is not worth a "preimum" anymore since most of them have abandoned service LOL!