The uair is an amazing link for three things in combos: 1.) Starting them, 2) Comboing in them/juggling, 3.) Moving towards the finish. It's good against all characters but in different aspects:
1.) Fast-fallers - Because you can use it as a link between combos/CG's because it's a follow up for a finish
2.) Weighted characters - Because you can shffl/juggle them across the stage if you perform it right
3.) Floaters - Because it keeps the ones like Peach in the air and abuses the fact that they has a bad DJ
What I usually do against FF'ers or Weighted characters is I'll get them to about 60% damage some kind of way (Combo, CG), then start to shffl uairs or just shffl one 'til they're about 80%, and then finish them(whatever's the best move for the finish ... it depends). It's effective man. Definantly something you should get down.
Oh yea, it's not necessary to shffl Doc's uair because the uair animation should be done by the time you hit the floor, but once the battle gets heatened up, you'll want to shffl them.