I spent a total of 11 hrs last month in the F lounges at both the A gates and the B/C gates. Food & beverage was the same at both. The lounge at the A gates was bigger and had more rooms for computers/napping etc so I'd hang out here over the other one given the choice. Both lounges were very sparsely populated though the whole time I was there.
I didn't visit the J lounges as I figured I'd go there next time when I drop to Gold.
I haven't been to as many lounges as most people here but the DXB F lounges are the lounges I've been most impressed with to date with regards to space, food, service and amenities.