The biggest improvement is going to be the radios. The 4.0.4 ROM its self has a few tweaks but with the new radios I have not had data drops.
I have always loved flashing new ROMs. All the ROMs out there are pretty much the same as one another right now, and all are based off of stock looking ICS with the exception being Miui (that I know of). Benefits of the ROMs is generally a faster updates,battery life,performance, and fixes for issues that might occur in a stock ROM.
I am currently on Gummy 1.2 with an imoseyon 3.7 kernel and love it. I have 31% left and am going on 27.5 hours with some light use since I have been home. Generally on a moderate to heavy use I am getting 15 hours, give or take a hour or 2 depending on usage. The day before I was at 17hr when I hit 15% and had 2hr 10min of screen on time.