The ring of spell storing is tailor made for warlocks! Woot woot!
Mostly at low level.
Once a warlock gets to level 11, all of his spell slots are level 6. Putting a 1st level spell from a 6th level spell slot into an item that only allows up to 5th level spell slots might not quite work. I suspect that the game designers did not consider this, and I suspect that many DMs would allow the Warlock to power down the spell slot, but technically speaking, the 11th level Warlock can only cast spells at level 6 since all of his slots are level 6 spell slots. The ring only allows spell slots of level 1 to 5.
The ring states "The level of the slot used to cast the spell determines how much space it uses." and "The spell uses the slot level of the original caster".
They never get slots above level five. They don't have spell slots for levels 6-9, but use the spell like power Mystic Arcanum instead.
... least likely would be five find familiar spells.
The ring of spell storing is tailor made for warlocks! Woot woot!
For a warlock, even one extra spell per day is HUGE. An extra spell per short rest (sometimes) is GARGANTUAN.Actually, if the Warlock get this at low level (levels 1 to 4), then he can have anywhere from 2 to 5 spells of his own spells stored.
Once he gets to level 5, it is only one 3rd level spell stored. So, it tends to be one extra spell per day at levels 5+ (although he could get an extra spell per day each time he has a spell left over before taking a short rest, or each time he takes two back to back short rests).
For a warlock, even one extra spell per day is HUGE. An extra spell per short rest (sometimes) is GARGANTUAN.
I don't think most players should expect to be getting rare items before level 5, but I'm sure there are plenty of exceptions out there. (Please don't anyone comment just to say you're the exception. I honestly don't care.)
Mage armor is easy but if you need it you likely already can get it.
It's not quite as huge if you have to pick the actual spell ahead of time.
So, it's not as potent as his own spell slots.
Getting one (and sometimes two) extra xxx spell a day is nice, but not huge.