Essential
If you are using the Minna series on your own as a self-learner, my opinion is that these CDs are indispensable to your experience.
I can't really separate a review of the "extra" CDs from a short discussion of Minna in general, so if you're interested in that, read on.
First, the standard disclaimer. There is a partisan divide between the Genki tribe and the Minna tribe. Saying something good about Genki or bad about Minna will provoke passionate reactions from the tribes. I try to be as objective as I can, but I realize we can be very subjective when it comes to our preferences. I haven't come across many Minna partisans, I must confess. Most "westerners" seem to prefer Genki; as far as I can tell, it's used in most North American universities, so the exposure of Genki is far greater in the West. Minna has, however, been used in Japanese programs like Columbia's and the SOAS's in London, so there's that.
Disclaimer: I'm a North American myself, and I own both Genki and Minna, but it's into Minna that I have put the greater time investment and commitment.
Secondly, my initial impression of Minna was actually off-putting. Minna is quirky and cumbersome compared to Genki, and although Minna lays it all out, it does NOT take a spoon-feeding approach. I think of Minna as much more of a "complex learning system". It has a lot of components, and a specific sequence that one takes through it all. The publisher does describe what this system is in the introductory material of the text and on it's website, so although it's not a "secret", for most readers it's probably hidden in plain view, as I know most people skip over prefatory information. I skipped over this information myself until I'd already got through many chapters. Regrettably, I learned about the existence of the extra audio CDs only after I'd made it half way through book one of the beginning text. Well, I'm just plodding along on my own here, and have been learning about the "system", in addition to learning Japanese, as I proceed. The publisher has obviously put a lot of thought and work into this system, and it requires time and effort on the part of the learner to do it justice, particularly if you do not have a teacher holding your hand. I think I've got the "Minna system" down pat now, and I think that doing so is fundamental to getting your time and money's worth.
Thirdly, I've read that people complain about the "tone" of Minna, that they think it's too serious or dull. I understand this is purely subjective and could be true for some people. It's not true for me; I think the Minna tone and approach is interesting and even "fun". But things can be "fun" without being sophomoric. It's true that Minna takes a serious approach to grammar, but serious is by no means equivalent to difficult or boring.
Lastly, I can certainly understand why Minna would not be a learning method for everyone. Minna is by far the best for me, but individual results will vary. If I had to recommend an approach for the self-learner, I would definitely advise people to give a serious look at Minna. Using Minna effectively, and I will emphasize "effectively", is the best way I know for a self-learner to get a solid introduction to reading, writing, and listening to Japanese. The extra audio CDs that are intended as companions to the main Minna textbooks are integral to an effective use of the system.
