Does anyone know if the 3x per day Improved Haste ability which the ring is supposed to bestow actually works? Every time I use it, the character doesn't look like they're invoking any ability and doesn't get the "footloose" icon on their portait. Only the number of uses/day left decreases by one.

If you are wearing a Ring of Free Action or something that bestows similar properties, that may negate the haste. Also, it probably can't be used on monks for some reason. What class is your character and what equipment do they have? Not your main character, but the one(s) having this problem?
-Brainless One

Korgan is wearing the boots of speed (since it's nice to have that much axe-wielding power arrive quickly). No ring of free action. Besides, I remember having this problem in previous games as well. Imp Haste and Haste usually stack, but here the Imp Haste doesn't seem to ever take effect.

Ah. The boots of speed will negate further haste abilities, if in fact Korgan is the one you are using the ring's abilities on. Test it on someone else, and if that doesn't work, check the item's abilities in an editor.
-Brainless One

He is and I will. Thanks. I thought they stacked, but that must be just plain old haste (no boots involved).

As far as I know, no haste can be stackable, unless it is something like a Greater Whirlwind (ToB) or likewise. I am not completely sure though... the fine limitations of items and spells are not my strong point. Hope your ring works out fine in the future!
-Brainless One

i'm finding this a bit tricky too with my solo archer (i 'hired' a thief to open the door ;) )

it seems to take a while to take effect and then is very short duration, and like you say, there's no icon or animation for it so its easy to miss and the action is easy to cancel if you don't wait long enough - i'm fairly sure i did get it to work though

all that is only based on observation though - don't take it as gospel (and i did have boots of speed on - that's worth testing)

<span style="color: lightblue">Generally, the speed bonuses from Haste effects will stack, but the extra APR will not.

The boots of speed and haste will stack. The movement rate will be doubled when wearing the boots of speed and doubled again if you drink a potion of speed, for example.