The Intranquility spell is, unfortunately, the first resort of many a rejected lover. In some ways it makes sense -- the more unhappy and forlorn one is about a breakup, the better this idea of making the other partner feel just as much so starts to sound. Unfortunately, this spell is often not well suited to a case. The purpose of the standard Intranquility spell is to have the person be tormented by the spirit until they make contact with you, or whomever the spell is being cast for. This means that if a person is already in good contact with their ex OR if they're one of those people who cannot restrain themselves from initiating contact, then this already is probably not the right spell for that case. If you've had an Intranquility spell cast and you make contact with the ex, then that's fulfilled the requirement -- the person can feel calm again because you've been in touch, and they don't need to do anything more to release the burden.
Another problem with the Intranquility spell is it is simply a rather mean spell. It's definitely not something to do if you're hoping for a happy reconciliation -- part of the spell even specifies that the person should be helpless and hopeless. Now, some people are probably deserving of this; and some people are just desperate enough to want this whether the target deserves it or not. Whatever the case, a lot of folks who use it claim they want the person to come back to them happy do so, and once again it's a bad move if that is really your goal. The spell isn't about making the person happy to see you. What success stories I've heard with the Intranquil Spirit, were stories where a miserable and obsessive person was returned, not a cheery lover glad to be back.
Additionally with the Intranquility, it is invoking an unhappy entity to go and torture someone. This spirit has a bit of a reputation for being difficult to control. I've heard more than one case where it was reported that the spirit actually went after the wrong person in the equation (the abandoned lover, not the abandoner after whom she was meant to chase) and there are also accounts of the spirit sometimes refusing to let up even after the couple are together again. [EDIT 9/2015: while it's true I have heard this reported many times, I have come to a different conclusion about the nature of the complaint. I now feel that the people who complain of this are people who were already inclined to be distressed and obsessed about their ex, and they are only blaming these feelings on the spirit.] In short, it's somewhat advanced work -- not good for a beginner who doesn't know what they are getting into.
Another often overlooked problem with the Intranquility spell is that it doesn't actually make the person love you or even necessarily want to be with you -- it just makes them feel bad if they are away from you. Honestly, many cases I've heard reported didn't work to ever bring back the target; all that happened was the target was made unhappy forever, but never decided to return to his or her ex.
In short, Intranquility work does not demonstrate itself to be more effective or more powerful than other return lover spells. In light of this, it should be used very carefully.
The Intranquility Spell is suitable to use when one doesn't care about the target's happiness now or in the future, doesn't mind working with nasty spirits, and had good control over their own behavior. As Cat Yronwode puts it: "If this is your idea of a love spell, ugh."
In the book Mastery of Love, Don Miguel Ruiz writes about how most people's ideas of love are rather screwy and based more around control than around the virtues we like to consider as affection. I've observed this is pretty much true. A lot of love spell cases don't pan out well, and this is often because people frequently act in contrary manners to what will help their case ("sabotaging their own work" as some practitioners say.) This often happens because control, not love, is really what the person seeks. I'm not going to tell society to change its ideas about love, I frankly am fine with it -- but understanding that much helps to understand how to work with things, and at that, Intranquility Spells still aren't great moves if you want control as it's all in the hands of a spirit, with you unable to make contact. These spells are not much for Commanding; they are for Revenge. I am willing to cast them myself as much as I'll cast any love or revenge spells, but I will say I'm picky about it. This story helps illustrate why I don't like getting caught in them.
However, for those still interested in Intranquility Spells, I will add this helpful information: most success stories seem to involve supplementing the spell with more benign love work, such as Come To Me or Honey Jar spells. I also find that the victims who respond best to Intranquility often have dark or abusive personalities themselves, and so really do perceive misery and harm as an aspect of love.
Additionally, my own experience is that one does not necessarily have to go all out by calling on The Intranquil Spirit. I have had some successes by just adding a little Restless powder to other love spells to achieve similar ends but much more gently. This makes the person anxious and restless till they see you, without complicating things by involving a mad spirit who is "in Hell and will not reach Heaven."