Community Day experience so far
The dratinis are as common as pikachu was last event.
Shinies seem common too. I haven’t caught one yet, but ran into a guy who had two in the first 30 min.
About half of my incense spawns were dratinis.
Lots of server lag and network errors.
Stardust is only doubled. You need to use the star piece to get 3x. Seems like a big miss by Niantic.
UPDATE: Just played another hour. Server issues much improved and caught 3 shinies. Stardust bonus was also fixed.
In total, I caught 106 dratinis in about 100 minutes of play, with a pinap for almost all of them.

Answers
Thanks for the info/preview. I was just spinning/fighting some gyms at 10pm Friday central US time, seemed to have a lot of lag/network errors. Gonna expect some more of it during our event hours. Starting with 205 dratini candy and 62 pinaps. I’d be pleased enough with 150 to 200 more candy. Stardust can happen all the time, not going to worry much about that aspect.
Update after the event ended:
Walked about 7 km in 1.5 hour. I got a total of about 40 Dratinis, 0 shiny. They were definitely not so clustered as in the Pikachu event, which took over all regular spawns. Remarkable was that almost every Dratini was below average. I evolved my previous wonder Dragonair to Dragonite.