Well, here's my experience for 2014 so far as a lowly Gold: 14/16 flights upgraded in January. Routes included CLT-DTW-CLT, 4 flights between GSO-CLT-GSO, 2x CLT-MCO-CLT, CLT-MEM-CLT, RDU-CLT and CLT-YYZ.
Only two upgrades I missed is one CLT-RDU mid-day flight (no big deal for the 25 mins. in the air it was), and one YYZ-CLT- also no big deal as I had a bulkhead with nobody beside me on the E175- tons of legroom, which with nobody beside me was more spacious than most F seats.
My flights between CLT and MCO were on A332 and 767 equipment. I was upgraded at the Gold window on the first trip, and saw an empty seat within 24 hours between these city pairs on my second itinerary which I was able to snag by re-checking in.
One caveat to re-checking in though- I found it only works at the beginning of the itinerary for that day, and not between segments. For example, if you're flying GSO-CLT-MCO and you try to change your seat on the MCO leg after arriving in CLT, the system won't allow it (even if you're already in F). It says you're already underway and only allows you to view your itinerary. Any further changes to seats or class need to be processed either in the club, at the gate or on the phone. At least this has been my experience so far. YMMV.