Chances are you do not need PACK_KEYS. I see you are using BIGINT for your PK. How many rows are you looking at having in this table eventually?? What kind of data are you storing? How do you intend to retrieve/report on it and how often?? These are things I would consider first before using this feature.
If I read that documentation correctly, it's basically stating that if you have two consecutive records with long PKs say:
PK-x: 1002350025789001 PK-y: 1002350025789002
With PACK_KEYS, PK-y now becomes something like "[pointer to PK-x]2"
It's basically a way of saying PK-2 is the same as PK-1 except for the last number which is 2... without having to rewrite/store the same refix/preceding numbers.
The gains from this are most likely only realized when you are dealing with very long PKs and will mostly be gains in storage/memory, however I would imagine there's a cost to overall performance which may or may not be noticeable depending on how much access load that table gets.
May not be worth it... I've never used this feature, and I've built some pretty heavy apps on MySQL.
hope this helps.
Good Luck