I don't have personal experience with naltrexone injections or implants, but of the people I knew who did it most of them hated it, because it does nothing to reduce cravings, merely blocks the effects of opioids if you use them. I have known people who stopped going for their injections, or dug the implant out of their arm with a razor blade. The only people I've known for whom it seemed to work were people who were very committed to staying clean, very stable, and who did a lot of things to work on their addiction like rehab, therapy, groups/meetings, lifestyle changes, and so on. I think under those circumstances it can help because it can make it easier to not act on your cravings if you know that you won't be able to easily get high. You have to get the injection at least once a month for it to work effectively. Some people have found that by taking huge doses of opioids they could over-ride the blocking effect of the naltrexone, so for people who are really desperate to get high they will sometimes just use larger and larger amounts of opioids (which can be dangerous as you can still OD).
I have also talked to some people who got side effects from the naltrexone, like insomnia, nausea, anxiety, loss of appetite, chills, difficult ejaculating/orgasming, diarrhea, dizziness, fatigue/low energy, depression, headaches, irritability, and pain. People also say that it blocks the natural opiates, endorphins, your body produces, making it difficult to feel pleasure. (I haven't been able to find any definitive research yet but a lot of people have complained of anhedonia and related side effects like not being able to get a natural "high" from exercising, sex, etc. - note this applies to high constant dose naltrexone like in the injection or implant, not low-dose naltrexone or naloxone pills).
In order to get the injection, they usually want you to have a minimum of one week clean from shorter-acting opioids, longer if you took any longer-acting opioids such as methadone or buprenorphine. They will do a drug test, and/or sometimes a test with naloxone and see whether or not it gives you precipitated withdrawals, before administering the first shot of naltrexone. It's really best to NOT have that "one last shot" of dope before going on naltrexone for a number of reasons, one being that it is very hard to have
just "one last shot" and it can screw up your recovery and make your cravings worse. I have heard that if you literally only use opioids for one day and are not physically dependent that you can have the naltrexone much sooner as it will merely reverse the effects of the heroin and not put you into withdrawals, but I do
not recommend you use before getting your Vivitrol shot (plus if you fail a drug test they will make you wait, then you might be more likely to use some more heroin).
In the study that was done to determine Vivitrol's effectiveness in preventing people from using heroin before it was approved for use in the US, they found that most of the people in the study given Vivitrol used heroin 8%-31% of the time during which they conducted the study (24 weeks). The participants had gone through inpatient detox, had at least a week clean prior, had to attend counseling and had sponsors etc.
(study here)
Note that naltrexone can also blocks some of the enjoyable effects of alcohol and you might get sick if you drink while on it.
ETA: If you get the Vivitrol shot (or take any form of naltrexone) I strongly recommend you keep a card in your wallet in case you are in an accident or something with a message to any emergency medical personnel stating that you are on naltrexone, what dose, when your last injection was, etc. This way, the clinicians in the ER, the anesthesiologist, etc will know to compensate for the fact that you are on naltrexone by using different drugs or much larger amounts than normal.
Sorry this is so long, I hope it is helpful