You can find the rest of the series here: Part I: Wording and Whatnot and Part II: Limiting Wishes.
I've only had a few wishes used in my gaming history. But I've given a number out in GURPS, in the form of a Great Wish spell. So let's talk about the Wishing spells in GURPS Magic and then what happened with mine.
In GURPS Magic, there are three Wish spells - Lesser Wish, Wish, and Great Wish.
Lesser Wish - all this does is let you dictate a die roll's result before you roll it.
Wish - all this does is let you dictate a die roll's result after you roll it.
Both are cool, but they aren't really that exciting as wishes. They can be exciting in play - my players burned through some in a pretty amazing fight against some really tough opponents. But a called "3" to the head is nice but it doesn't have anything of an Aladdin-like feel to it. My players really like Lesser Wish, since tactically being able to call a critical hit on anything you can get a 3+ to hit with is potentially fight-turning. But again, it's pretty mechanistically "called luck" and not "magical wish-granting."
Great Wish - now we're getting somewhere. This spell can permanently add to your attributes, give an advantage, add to skills, or remove disadvantages. It can do the same to your enemies, too, if they are present. It can also replicate any spell at all, as if cast at 1000 energy (half of the cost of the Great Wish) - which is more than 3x as much as the next most expensive spell I can find in the book, Resurrection. So it can really cast a hugely powerful spell (by trading energy for skill) or a really big area spell (by dint of sheer energy expended.) It can also do "absolutely anything else that the GM feels will not ridiculously unbalance the adventure or the campaign!"
Pretty cool. But it's basically not castable. It makes a magical item, which anyone can then use, but the chance of a permanent power grant is hard to resist. This is especially true if what you ask for is something you cannot get by any other means. Great Wish is the ultimate excuse to suddenly be able to fly or be a Weapon Master or become a wizard. No one every had one of these and used for anything except to gain some kind of power. One spell that could do anything is nice, yes, but when it is so important to cast such a big spell that it's more useful than a permanent new ability you can use session after session? How about a shot at asking the GM for some crazy effect, knowing what the limits are for duplicating spells and for granting/removing traits?
Yeah, exactly. Everyone goes for the permanent grant over the temporary but decisive usage. No room for argument, it says right here I can give myself +1 DX with it, so that's that.
There is also a Wishing enhancement for things like Serendipity or Super Luck in Powers. But since the ability to have lucky events happen to you isn't terribly strong, you can't really grant too much gain. There are ways to amp this up a lot, but again, it generally comes to down to giving someone better rolls, creation powers to make things, movement powers to move them, etc. and a pile of points (GURPS Powers, p. 119, has examples - be prepared to spend hundreds of points for some pretty narrow effects.) I haven't seen a lot of this used in play. If you do use it, the results are even more specific - no wishing someone dead from the guy who has Luck that works on others or can create object for you, it's just not covered. This can be good or bad, depending on how you limit wishes.
In my previous, sprawling fantasy game, I granted two wishes apiece to the PCs, albeit with a lot of time between the first and second . . . and they knew the second could and would come. Like I said, I granted these - well, a pair of magnificently powerful stone heads granted them - as a power graft onto the PCs to help them fight the Big Bad Evil Guy.
I made people request the results in plain language, which gave me some wiggle room, and admittedly let me squash more munchkinly requests (like the guy who wanted Magery 1 plus 2 additional levels of One College Magery and tried to work that into a non-game terms wording.)
The first batch included getting Trained By A Master, Danger Sense, +1 to DX, Luck, Wild Talent (one unlimited, one spells-only with retention). The second batch was a bit more varied, since the players realized they could ask for more and (if they had points to spend) could exceed the limits of the spell a bit. One guys asked to be a better demon fighter, and I crafted a Demon Hunter meta-trait for him. Another got an Energy Reserve and a special Leech power. Another got the ability to ignore some spells on (which later became a more limited Perk in Magical Styles), still another got Heroic Archer (making him the first), and one got a big jump in Charisma. One even got a limited Invisibility power.
Even so, and even with this granted specifically as a power grant to the PCs and a chance to buy new stuff, it took a second Wish each before people really got into the idea of using it to ask for wild and crazy powers.
To be perfectly honest, I find the wishes in GURPS Magic to be a bit mechanistic. Cast the spell, and it lets you do something pretty specific. Even the limits of Great Wish are so specified out that I've found it limits what players will try to do with them. Instead of just giving guidelines, the terribly specific nature of what it can do means no one really tried to do anything terribly interesting with it. It was just a convenient way to get some extra power on their PCs, and get things they might not have been able to get otherwise.
In my current DF game, I haven't given any Lesser Wish or Wish items, nevermind a Great Wish item . . . but there are some Great Wish (or at least Great Wish-like) items out there. I fully intend to being a bit more generous with that "absolutely anything" clause and a lot more restrictive with the "power grant" clauses. After all, people can already gain all sorts of crazy powers just with points in my games. If they get their hands on a wish, I want it to be a bit more magical and awesome, not just a chance to get +1 DX without saving up 20 points for it.
I can't confirm or deny, yet, if there are also lesser versions of the Great Wish spell, or if they use the "redistribution" aspect of wishing I mentioned before. Or if all wishes hang together, or come apart all together or not. But they might, they just might . . .