It's huge for the warlock because of they way he plays. Warlocks are all about using the same spells over and over again. You could load fireball or blight or blindness or hellish rebuke or hex into this ring and basically increase your casting power by 50% if you are less than level 11 (and by 33% if you are levels 11 to 17. It actually lets you cast more spells in a day every time you short rest, whereas it only helps normal casters port a few spells over from the previous day. Of course, it prolly helps none casters the most. It is also one of the only ways to get multiple concentration buffs going. Allowing the great weapon master to concentrate on his own bless spell is awesome.
Sorry, but it rarely increases the Warlocks casting power by 50%. It typically only does that in one encounter days (assuming the player knows it is a one encounter day and is not saving one or more slots for later encounters), or in days with one encounter in the morning and one many hours later (where multiple short rests can be taken in between) and it only does it when the Warlock is using up all of his spell slots and his ring before each set of multiple short rests.
In normal adventuring days (like in a dungeon), the lower level Warlock would need to cast all 3 spells (the ring spell and his 2 spell slot spells) in the first set of encounters, followed by two short rests, followed by doing the same in the next set of encounters, followed by two more short rests, etc. to get to the 50% rate. I don't see that happening too often in too many groups.
Warlocks are all about using the same spells over and over again, not same spell over and over again.
If the Warlock puts Fireball into the ring and Fireball isn't applicable in a given encounter (or group of encounters), then neither is the ring.
And how does a short rest help? If after the first encounter of the day, the Warlock uses up both of his spell slots and his ring slot, then he takes a short rest. He only gets back his normal spell slots, not the ring slot.
It is only in scenarios where he does not use up all of his spells slots and he has one remaining at the start of a short rest where he can go fill up the ring (or in cases where he is able to talk the party into doing two back to back short rests).
At most, this is one or two extra spells per day (three on a great day). In the big scheme of things, one or two extra spells per day is not huge when in 5 encounters with two short rests, the lower level Warlock is already casting about 6 spell slot spells per day. It's a 16% to 33% increase in number of spells cast per day assuming that the spell put into the ring each time is the one needed. At levels 11+, it's 1 or 2 extra spells per day when in 5 encounters, the Warlock is casting maybe 9 spell slot spells per day, or 11% to 22% increase.
If the spell in the ring is the one needed, the Warlock should always cast out of the ring instead of using up one of his slots. But 1, 2, or maybe even 3 spells per day when the Warlock is already casting upwards of 9 or so spell slot spells plus many other magical effects each day is helpful. Helpful, but not huge since the Warlocks only source of spells is not just his spell slots. We're talking less than 10% of the overall magical effects that he does in a given day. In a few encounters, it does give the Warlock an additional spell, but it won't be every encounter.
It'll tend to be 1 or 2 extra spells per day since the Warlock will rarely have the chance pre-short rest to fill up the ring (because if the PCs are taking a short rest, the Warlock will rarely have a spell slot available to fill the ring up with).