Once you get the correct FFB settings for RealFeel, it does make a huge difference to the game. You don't get the unrealistic feedback you would get with other racers - RealFeel literally translates what the car feels at the steering rack directly to the FFB.
Of course this means the mod needs to be written correctly in the first place - some of them I think don't concentrate on realism at this level - so much like everything rFactor: it's mod dependant.
Shop around on rFactor Central and pay attention to the comments and ratings for individual mods. Most of the really popular ones take RealFeel into consideration - I wouldn't go back to anything else now - I like the almost raw feel to the FFB which takes a little getting used to, and can be toned down with a simple edit of the config file.