PSA with math: The secret stash keystone is not bad.

I've heard people speak negatively of Secret Stash because they feel it doesn't provide enough of an impact, how manaless champions don't get enough from it or how the other tree keystones are better.

Math time.

Potions recover 150 health over 15 seconds ticking every half-second (5 per half second, 10 per second).

Secret Stash increases the duration by 10%, up to 16.5 seconds.

(10)(16.5)=165 health.

Now let's add the +20 initial health it gives you.

165+20=185 health

Now let's see how large an impact it has total: (New amount/old amount) 185/150=~1.23

===

Even if you don't use mana, you're getting 23% increased effectiveness out of your potions; you're getting 11.5g worth of healing extra for every potion you take. If it doesn't cater to your specific playstyle, that's perfectly fine -- not every mastery is going to appeal to each and every playstyle -- but it is by no means "weak", especially compared to the other first tier keystones.