Given the nature of the Welsh diaspora, learners probably fall under one of two categories: those near or in Wales, with adequate provision for supplementary learning (talking to Welsh-speakers, joining a club, etc.) and those far-flung, isolated people across the world who wish to learn something of a once native language. To this second category belong people with little or no Welsh heritage who are fascinated by the language. Given perhaps the use of English as a medium, and what I would think would be a certain lack of Welsh-learning resources in other languages, to this category also belong non-L1 speakers of English who want to learn Welsh. Given the nature of the language (very different from other standard languages taught in Anglophone schools), a solid beginning is a necessity. It is very easy to get discouraged early on--so much is unfamiliar, there are odd things like mutuations to get used to, and the syntactic structure is strange.
This is a worthwhile text to begin with, as the pacing is just right at the opening and the voices on the CD are really like real people (I could list some characteristics but will spare you--the people attached to the voices might be reading this!) This is engaging, and the grammar is introduced slowly and just right. There are of course other texts on the market, and I have tried one or two (but, as I just said, got discouraged); this is the best. But it is wise to buy the CDs too--the language is relatively difficult to pronounce at first.
There are a couple of more technical issues which might be addressed if and when there is a new edition. The internal margins of the book are very narrow, which means that using the book, flipping back and forth, going over pages three or four times, I can see the binding coming loose. The chapters on the CD correspond to those in the book, rather than the dialogues within the book chapters, which means that, on an ordinary CD player, it is time-consuming to find an individual dialogue mid-way through the chapter. (Other Teach Yourself language books do this differently.)
Final point: there's also a Teach Yourself Welsh Grammar, which might be considered alongside this book. I at least have found it quite helpful.