First, a little background: I've done the Pimsleur French course (all 3 levels), as well as numerous other materials, audio, text, and software. So this is not my first course of this type.
On the whole, this proved to be excellent for speaking practice, with the following caveats:
1. Michel Thomas' approach is to teach two new students, and you listen and imitate. But the inclusion of new students in the audio program means that the listener hears not only Michel's French, but also the mistakes of the students. For me, this was not a big deal, but had this been my first experience with the French language, it probably would have been.
2. Michel is not a native French speaker. He was born in Poland and lived most of his early life in Germany, and his accent is obvious in both English and French. Again, if you have some prior French language experience, this may not be a big deal, but I would certainly not advise anyone to imitate Michel's accent.
3. In addition to the students' mistakes, Michel himself makes numerous grammatical and pronunciation errors (aside from his accent) over the course of the program. For example, he repeatedly states, "Personne est..." where the correct form would be "Personne n'est..." he even states explicitly that the "ne" is not used if "personne" begins the sentence, which is simply not true. He also states that "un" and "une" are pronounced the same in rapid dialogue, which he describes as a "brief touching of the n," but the "n" in "un" is never pronounced unless the following word begins with a vowel. He also states that French words are accented on the last syllable, but this is also technically incorrect - officially there is no stress accent in French, and what stress does appear in actual dialogue is very loose. Once again, if you are at least somewhat experienced with French, these issues may not matter particularly, but if you are new to the language, there are a lot of bad habits that you could pick up from this course.
Aside from all of these problems, this course does provide quite a lot of conversational practice, and the vocabulary is markedly different from that of the Pimsleur course, which for me meant a lot of opportunities to learn new words and refresh vocabulary that I'm not used to using very often. Michel does go through a lot of vocabulary very quickly, though, and he definitely does not review to the extent that the Pimsleur courses do, so if you are starting out in French I would recommend beginning with the Pimsleur course if it's available to you (check with your local library - they frequently carry them, or can order them at least). Then, once you've got the basics down, you might give this course a try.