The Soda Popper is a beast with the pistol. Imagine a pistol where every shot deals 20 damage. My god. And when I want to mess around it makes minicrit bleed with the Boston Baster a breeze. No more running away to a safe zone, drinking crit a cola and having your opponent take like five more damage than normal.
That's exactly what I used to do (the Popper + Pistol + melee emphasis combo, not the Cola then immediately die bit), but the way it ended up I wasn't actually ever using the Popper. Alone, it's a Force-A-Nature without its major gimmick and with marginal extra practicality; decent against something you can shoot from a foot away, useless against everything else. There's not even an incentive to use it because the crits build up so long as you're moving. Aside from when you've got crits, the pistol never does enough damage for anything other than mild harassment and taking out unattended sentries (also remember that firing it for anything longer than a half-second makes every shot go wild). As far as my playstyle was concerned, it was a wasted slot.
The shortstop's only apparent weakness is that you can't do as much damage when sneaking up on someone than you can with the regular scatter. I'm doing as much damage as I can with the Direct Hit per shot at medium range, with a hitscan weapon, with a far faster firing speed, and a far faster reload speed...
Current loadout is Shortstop (obviously) + Bonk + Sandman or Basher (depending on map and enemy team composition).