(EDIT, 6/20/17: Up to date information about the Intranquil Sprit can be found in my book The Intranquil Spirit, available on Amazon. This post has some incomplete information which is clarified in the book.)
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."
As I've said before, it is a curiously popular spell -- perhaps because the desperate who wish to bring back a lost lover enjoy the idea of making their mate as tormented as they are. Some people who cast the spell for themselves even find they feel the spell to have "backfired" on them and claim that THEY, instead of their would-be lover, were the subject rendered intranquil and unable to think of anything else but their desired person.
I don't have any great moral objections to the Intranquility spell: I just don't love doing them because I find their reputation to be rather ill deserved. They just don't work all that well, in my observation: I have seen it to be one of those spells where even if it works, it doesn't really work -- that is, it doesn't do what the person obviously had intended to achieve. Even in cases where everything goes perfectly -- the person who wanted the spell stays quiet and doesn't contact the target of the spell, where there's no backfiring or weirdness, and where the target doesn't just hold out for a lifetime of intranquility -- what generally seems to happen is that the target is rendered intranquil and depending on circumstances, either contacts or comes back to the person who wanted the spell cast. Things are then hunky-dory for a short while, but once the (usually male) target feels contented again, he leaves again. Working with the Intranquil Spirit is, let us say, not a one-time affair.
But as I said in my last post on the subject, it seems to be almost as common for the target to just hold out for a lifetime of strife and stress, based on the reports I've heard from those who used the spells and were able to keep tabs on their exes. It is a general matter with love spells that they cannot go against a person's free will: and no, that's not saying there's some universal rule declaring it's immoral to force a person's free will, I'm saying that love magic spells -- unless the person being targeted is extremely weak-minded and incapable of judgment -- are not able to turn a person into a "zombie" who has no free will and no ability to discern if the course of action being proposed is unreasonable*. This is, for example, why love spells on celebrities whom you've never met don't work: Brad Pitt knows perfectly well, just by way of rational thinking, that it would be a dumb idea to drop what he's doing and go scouting around rural Idaho for some girl he had a dream about (for example) just because he's having bizarrely fond thoughts for this mystery person. Likewise, your ex may know that he just cannot deal with you anymore, and know that even if he's having remorseful or impassioned thoughts that life with you is just not something that's logical any longer; and so he'll ignore the calls from the love spell and keep about his business. This includes Intranquility spells.
As is warned by Jaya over at her blog, "The Anima Sola and Intranquil Spirit don't necessarily reunite lovers so that they are married and live happily forever amen but may reunite lovers only for a short time. After that time is over, it will be very difficult for you to find another love or even have sex with another person as such a strong love spell will certainly create a sexual dysfunction for you other than your former beloved."
Intranquility spells can of course be used by anyone, but they ought be done with much caution.
*And I just love it when people insist "No, it's totally possible! I know someone who did it!" Of course, if this is the truth I want to know all about it! Unfortunately, this story never holds up to further questioning.