Michel Thomas is almost as good a language teacher as he is a boaster, and that is saying a lot! Don't be put off by the boasting - this course really has a lot to offer, despite some serious flaws. I'll list the main strengths and drawbacks, as they seemed to me.
Strengths
1 Thomas concentrates on structures, especially the verbs in their various tenses. We really do learn very quickly how the language works.
2 He teaches these structures quite systematically, despite his apparently rambling style.
3. He explains things simply and clearly.
4. He involves us all the time - there is no passive listening.
5. And we really do (OK, I really do) remember what he has taught us.
Drawbacks
1. His Spanish accent is not good - especially the way he stretches out stressed vowels. Every native Spanish speaker I've heard makes them short and sharp. This course needs to be supplemented for pronunciation by recordings of native speakers. But this is not as big a problem as it might seem. Spanish pronunciation is not difficult - above all, the relationship between pronunciation and spelling is incomparably more simple and consistent than in English.
2. His teaching method consists entirely of getting us to translate sentences, which he provides, from English to Spanish. There is no practice in listening to Spanish, no conversation in Spanish, and indeed no opportunity to produce Spanish speech of one's own. But I find it much easier to go on to practice and engage in conversation and speech production now that I have Thomas's basic structures firmly in my mind.
3. The vocabulary we learn is very limited. Again, this needs to be supplemented from other sources - such as phrasebooks. But the phrasebooks are much more useful in the context of the structures we have learnt from Thomas.
4. Thomas teaches Latin American Spanish. This is not in itself a flaw, and he does mention some of the differences between this variety and that of most of Spain - eg in the pronunciation of the soft 'c'. But he omits to point out that in Spain (unlike Latin America), verbs have a second-person intimate form in the plural(with vosotros), parallel to the second-person intimate form in the singular (with tu). Thomas uses the 'ustedes' form for all situations in the plural, intimate as well as polite, without mentioning, let alone teaching, the 'vosotros' form.
5. He doesn't cover all the tenses and moods in this course - particularly the subjunctive, which is left for the 'Advanced' course that builds on this one, and is even better. (Avoid his 'Language Builder' course, though, which is difficult to use and adds little or nothing to this one.)
6. Finally, many people have been irritated or worse by the male student on the recordings, who is described by several reviewers in rather derogatory terms. In fact, he seems to me to be every bit as quick a learner of structures as the female student, though he unquestionably has problems with his pronunciation. (My guess is that he visualises the written word from Thomas's pronunciation, then tries to 'read' that written word in his mind, rather than directly copying the sounds that he has heard.) But this really need not and should not be allowed get in one's way.
All in all, then, this course is only a beginning to one's study of Spanish. However, I think it is an excellent beginning.