I've never shot full frame. APS-C was the format I started digital using--a Canon 30D. I now shoot Fujis and a Ricoh GRII, still APS-C, and an Olympus Micro 4/3 on occasions. There have also been a few pocket cameras with smaller formats.
What I have noticed is that image quality has improved markedly in the formats I've used over the years. I was an early adopter of the Micro 4/3 format and I also used Olympus's standard 4/3 DSLRs. Comparing those early 4/3 images to more recent ones, it's obvious the more recent photos are sharper with better tonality and lower noise. Comparing the early APS-C to more recent ones shows the same improvements. Some of this has to do with lenses. I'm using better lenses now than I did early on. Fuji and the Ricoh's lenses, as well as the PanaLeica M4/3 lenses I now use have the sharpest, most satisfying look to them of anything I've ever used. Today, I can see little difference between recent M4/3 and APS-C when optics are not included in the equation.
Full frame is tempting but I can't justify it. My largest prints are cropped images on 13x19 and even the older digital images print fine at this size. I shoot most of my photos in daylight or under bright artificial light so noise and dynamic range is not really an issue for me. Anyway, I used to shoot for a daily newspaper so I know what pushed Tri-X looks like and it can't compare in quality to high ISO digital. I don't think full frame offers me much improvement.