Well,the comfort improves greatly, and so does the headstage. The drivers are placed a bit further away from your ears, so you get less bass, and they sound a bit bright, because of that. Maybe a bit of foam in front of the drivers can remedy that (haven´t tried it). The midrange gets a bit recessed, and you loose the "in your face" sound, so instead of being placed at the stage, you move back to the 10th row.
They still sound good though, just not entirely like a Grado (maybe closer to a GS1000?).
Not the only one. I tried the G-Cush with my Grado 325IS and liked them for a while. The sound-stage was noticeably improved. Comfort was much better with the G-Cush. However, I went back to the bowl pads and haven't gone with the G-Cush since. The bowl pad sound is more "in your face" and that's why I got the Grado's in the first place. I'd like to try the HD414 pads but like "Reckless Fable" said, I too can't get past the yellow color.
I found the RS1i sounded terrible with G-cushions, way too bright and sucked out mids. I imagine the SR225 to be even worse since it's harsher than the RS1 to begin with. Comfort improves greatly though.
Well, I don't have to use my imagination. I have 325is with G-Cush and compared directly with the GS1K (not the i) and got pretty close to the GS sound. I don't like the stock pads that come with the 325is, uncomforable. I also own 414 headphones and I don't find them very comfortable either. So, there you go.