HIIT (High Intensity Interval Training), which can be translated as High Intensity Interval Training.
My opinion:
When it comes to exercise, is HIIT a good idea? Yes, as an exercise routine that is both more effective and “doable in less time” per session, thus being able to optimize your week and make it easier to include and maintain the fundamental regular physical exercises.
Does it really work? When well indicated (for those who can really do it and in the right quantity/quality), oriented and executed, yes.
Careful consideration of the issue:
1 - I went to Google and most of the sites I found approached the subject in an overly sensationalist way, promising “worlds and depths” and saying that HIIT is the solution to all your problems (which, of course, it isn't). Beware: the more sensationalized something is, the more it usually deserves your critical analysis before you apply it to your life! Among the sites, the best I found was this one (sensationalism aside): https://www.senhortanquinho.com/hiit-treino-intervalado-de-alta-intensidade-como-fazer/
2 - Never exercise without first checking your conditions with a good doctor (good anamnesis, complete exams, a qualified professional, up-to-date, well-founded, with good practical experience, really interested in your case and, above all, human), especially when it comes to HIIT which, as the name implies, is “High Intensity”. Not everyone can start HIIT as many people say, “straight away”, without running serious risks of various injuries: simply because their bodies are possibly barely prepared for everyday effort, let alone higher intensities! Take it easy and gradually increase the intensity as your fitness progresses.
3 - Be guided by a good physical educator: trained, up-to-date, grounded, with good practical experience, really interested in your case and, above all, human. When it comes to HIIT, I've seen a lot of people out there who seem to have a lot of “baggage” but who are so arrogant and lack a sense of cooperation and multidisciplinarity that they end up hurting their patients/clients. Humility and teamwork always help!
- This applies to all areas of healthcare: if your healthcare professional is one of those who thinks that his or her role is the only important one (or the most important one) for your results, I suggest that you run away from him or her because he or she may have difficulty working as part of a team and the damage will be all yours, as a patient.
4 - Physical exercise only works properly (and with a lower risk of injury) when it takes place in the context of a clear improvement in all healthy lifestyle habits, since the human organism depends on the simultaneous balance between several components, all of which are necessary. Learn more about HIIT, if possible, here: https://icaro.med.br/15habitos/.
5 - What happens in the studies and papers on which HIIT is based may not be what happens or will happen in your body: you may be a normal person who is not a professional athlete and/or leads a stressful life and/or doesn't have an excellent hormonal balance and/or doesn't have all the right lifestyle habits and/or has illnesses being treated and/or uses medication; in other words, HIIT “has everything to work” but in your case, due to various different factors, it may not even be indicated or require various adaptations: hence the importance of good professionals accompanying you, who are really interested in you and your health and not in validating methods and protocols as their primary objective. After all, you are not an “object of study” but a human being who wants help to live better, within your means, including time, right?
To sum up my opinion: HIIT is excellent (and I sometimes train using this methodology myself) and it usually brings results, but only if you follow the tips above correctly. It's as simple as that.
Okay? I hope I've helped!
Best regards, Dr. Ícaro Alves Alcântara
Português do Brasil 
English 