Bags are comfortable (for the most part), but I think that anyone who has taken a knee while wearing one would likely gripe about the zipper placement on the thigh pocket. It sucks. The FROG, or Fire-Resistant Organizational Gear, not only doesn't have that problem, but I actually find the material it's made out of to be a bit more breathable. Not to mention durable.
While in Afghanistan, my Marines and I were issued (1) bag each for convoy ops, and we eventually 'acquired' some FROGs from the grunts, who had ordered too many. Wearing the top under body armor is infinitely better than either cammies or a bag, thanks to nothing on the torso of the shirt to be pushed in by (in my case) over 70lbs of armor w/ gear attached to it. The pants are breathable, durable, have those spiffy calf pockets, and they take a lot longer to fully develop a disgusting field funk.
Clux is right about Marines and FROGs, too. There's either a MARADMIN or MARCENT directive (I can't remember which) that specifically states that the uniform for Marines conducting convoy operations will be the FROG.
Plus, you can take a shit in a FROG without having to strip your armor, helmet, etc. off.