I wore one of these CMI Integrated Magazine Pouches on my duty belt for several years and liked it, both in a regular uniform and Class Bs (this was after I'd left full-time police work and was carrying a commission as a Reserve). It was an easy way to have a second magazine handy without having to use a double-mag coupler or butt-stock pouch (they have their place and some folks like them, but I just didn't care for those options). Despite some of the ribbing I took for it, my concern wasn't so much to have an additional 29 rounds handy (though that is a Good Thing) but more because I was worried about having to conduct a reload in the event of a mag failure or weapon malfunction...perhaps a little of both.
Most users seem to either love it or hate it for its intended purpose (I know a couple of left-handed shooters who didn't care for it, largely because of issues exiting the vehicle if they were driving) and apparently some officers have begun adapting it for other uses - such as attaching a Taser or other 'ECD' (Electronic Control Device) to their belt for support-side draw (most places I'm aware of mandate a support side draw or strong-side crossdraw to prevent muscle memory from kicking in and causing a firearm instead of ECD draw). Here's a look at such a rig:
More at Center Mass Inc.