I always wear a IIIa on patrol. Generally on details, I step down to a IIa like the rest of patrol (vest-wearers, anyway). Wear is optional at my agency. I do not wear a vest on tubing (river) details, in case I have to jump in and swim. I keep a UDT floatation vest handy there (non-ballistic). I wear the IIIa for any court-related details, as I feel the statistical risk is almost equally high as when patrolling.
In the trunk, I have a plate carrier with SAPI's and IIIa plate-backers. It has rifle mags (for my Colt Commando clone) and a medkit on it as well. I bought all that, but was issued the IIa and IIIa concealable vests. (Everyone is issued a IIa, but my chief noticed I wear mine religiously and hooked me up with a great-fitting IIIa.) How they fit you greatly affects the feel and "bulkiness," or lack thereof, that you experience. Obviously IIIa is a little thicker/hotter/heavier. It sucks in the summer, especially in southeast LA, but I never leave home without it. No, I haven't been shot at, but I always go home.