Is it worth using the Fire Keeper Soul to upgrade the Estus Flask?

I found a Fire Keeper Soul in Undead Parish. Later, I went back to Firelink Bonfire, and a NPC offered me to upgrade my Estus Flask in exchange of the Fire Keeper Soul. I didn't pay attention, and accepted.

Now, I can see a +1 sign near my Estus Flask and it seems to give back somewhat more health. So do the Fire Keeper Soul item have other uses? Are they better or worse than upgrading the Estus Flask? How many time can I upgrade the flask, and how much do each upgrade change the HP regeneration?

Solution 1:

You can consume a Fire Keeper soul, and doing so grants you five additional units of humanity. I highly recommend you use them to upgrade your Estus Flasks however, as they are rare, and this is the only way to upgrade your flasks. If you don't upgrade them, the flasks will be of much less use later in the game since you'll (usually) have a lot more health to replenish. Here is more information on the Fire Keeper soul.