Any issues with hot coals falling through the lower vents?
Assuming the PK original, I have seen guys use a foil pan underneath it to catch falling ash. The PK360 eliminates this by redesigning the lower intakes.
How hard is it to clean out the ash?
Again, for the original PK, let it cool and dump out the ash. The capsule is aluminum, so you can also hose it out. The PK360 added a clean out plug and hooks below it to catch the ash. Remove the plug and use a stiff brush to brush the ashes out the hole.
Are the grill grates necessary/worth the extra cost?
It honestly depends on what you are cooking. I have had Grill Grates for 6-8 years and love them, so for me, yes, they are worth it. Can you cook without them, Yep!
How hard is it to add more charcoal during a cook?
The grate on both the original and the 360 is hinged, so if you are cooking indirect, build you fire on the side that lifts up, and if you need to add more fuel, just lift up the grate and add it. Actually, when cooking indirect on my PK360 I don't even put in the hinged portion of the grate. I just leave it out. For grilling, I haven't had a need to add more fuel.
Anything else you can think of would be appreciated.
The PKs are great grills and can double as a smoker/roaster for indirect cooks. Because of the generally rectangular shape it may not be the best for a long piece of meat like a brisket, but there are third party charcoal baskets that will let you set up a fire along the long side of the capsule if you want that.