I got a G25 a month or so ago, it was a big change going from controller which I've used since GT1.
I wasn't used to the feedback, especially as the last wheel I'd used (not much) was an MS sidewinder, which had no ff at all. I had a lot of trouble staying on the road so immediately put the FF on 1 and power steering on.
Since then, the car of my choice to try and control has been the tuned Elise, around Suzuka. Everytime I tried, I seemed to hit a wall in lap times, unable to go better and near unable to do anything in that first corner. I did make a few improvements, a second here and there but it wasn't going better, though I was having a lot of fun.
I have the G25 and a playseat in my brothers room, as mine is a bit small for that. I did need to play more though, so i got a DFGT and Wheelstandpro, i never changed the FF settings and my first lap was a second quicker.
I went to do some testing on the G25 and put the FF up to 6 or so and took the power steering off, within 10 laps I had taken 5 seconds off my time, and now I can see where time can be gained, I feel it on every corner.
Come to think of it, having the FF so low made the car feel like ice and my inputs were more of a guesstimate. I never knew it at the time though, and now i see what a difference FF makes.
Now i have it on 10, and doesn't feel enough
