I have a low-profile, small chest rig set up for use with my patrol rifle. Carries 3 AR mags, a BOK, a small dumper, and also has an admin pouch with a few lightsticks, Sharpie, spare 123s, etc. I considered something slightly more substantial, like the Eagle line of chest rigs, but I wanted to stay as small, light, and compatible with a Class A uniform and duty belt as possible. Having used it recently at a patrol rifle course/requal and a 3-gun match, it works fairly well.
SOP requires us to carry our rifles in the trunk until deployed, so I leave the chest rig in there with it. If I were going to a hot call, the rifle and rig would likely get thrown in the front seat.
I agree with you about the bag. I had a 5.11 Bail Out Bag before, and it was way too floppy and bulky. Going prone was also a bitch.
Here's my rig. Sorry the pic is upside down: