Michel Thomas Portuguese
(Burghead, Elgin, Scotland)
I can thoroughly recommend the Portuguese course. Having listened to Michel teach Spanish (and I do speak Spanish already), I was disappointed on two counts; one, was his atrocious accent, and two, he taught South American Spanish, not Castellano.
The Portuguese course, however, is completely different. It is presented by Virginia Catmur, an English lady who speaks excellent Portuguese, and who explains the lessons with great humour and patience, encouraging two students to figure out for themselves the construction she is aiming for, and then invites a native Portuguese gentleman to speak the word/phrase/sentence in an authentic Lisbon accent.
The two students, by the way, are a male and a female, have very different language abilities. You may well become exasperated with one of them (I shan't tell you which one - it will become obvious after about disc 2!).
I have the full set of CDs - from the foundation to the advanced, 12 discs in all - and it does take you from knowing nothing to being able to converse with portuguese people in their own language, albeit slowly.
I do have a caveat, though, and that is the advanced course is still what I would consider "colloquial" level, and you do really need to augment the course with another read & write course to get a broader understanding.
From my point of view, already speaking Spanish is both an advantage and a disadvantage. The advantage is the obvious similarity in the two languages, with a huge common pool of words.
The disadvantage is, Portuguese accent and inflection is completely different, and can catch you out at every turn. It may be easy to read Portuguese as a Spanish speaker, but the Michel Thomas course has been invaluable to me for getting my accent and inflection more on track, and steering me away from merely trying to speak Spanish in a Portuguese accent!