4 times more candy for the buddy function is huuuuuge. This is so very helpful, mainly for lapras and snorlax.
jigglypuff..... 4 candy for 1 kilometer, and my city currently has a nest of these. So you walk 10 kilometers and catch 5 of them, that is 40 buddy candy and 30 from the 5 caught ones, enough to evolve one and power it up a bit. So the pink army to level up gyms is real, its realistic to get the candy to evolve one per hour, and enough extra candy to push the ones that get a good moveset.
Or if you catch 30 pidgeys and send them away, that is 240 candy, enough to evolve 20 pidgeys, which gives another 20 candy. (The one candy you get back for evolving is the one thing not affected by the bonus). Obviously you first evolve them and then send them away. So you can evolve a bit more than 2 out of 3 pidgey, weedle, and caterpie, with lucky egg at 1000 xp each.
Then, double candy for whatever you hatch from eggs.
Over all, the impact is massive, and the warning time is a bit too short to save up arena gold for this. So i bet that many people will spend some money to buy lucky eggs, incubators, and maybe pokemon storage upgrades. Its just soooo effective.
I really need lucky eggs and incubators for this, maybe even incense.
If you play a lot, you can (In terms of XP, evolutions, and farming rare candy) make as much progress as you would otherwise make in a month in just those 5 days. Easily.